Recent developments in hybrid iron oxide–noble metal nanocatalysts for organic reactions

Article abstract of DOI:10.1016/j.cattod.2016.01.030

A review of recent developments in the synthesis of hybrid iron oxide–noble metal nanocomposites and their application to various organic reactions is presented herein. Focus is placed on various strategies for achieving (1) Au nanoparticles (NPs) on Fe₃O₄@polymer catalysts, (2) optimized dispersion and stability of Fe₃O₄/Pd catalysts, (3) Au NPs supported on Fe₂O₃–graphene oxide hybrid nanosheets and (4) rose-like Pd–Fe₃O₄ hybrid nanocomposite-supported Au nanocatalysts. Further application of such hybrid nanocomposites as catalysts for various organic reactions is discussed in brief.

Full text of DOI:10.1016/j.cattod.2016.01.030

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Recent developments in hybrid iron oxide–noble metal nanocatalysts
for organic reactions
Hyunje Woo, Kang Hyun Park∗
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A review of recent developments in the synthesis of hybrid iron oxide–noble metal nanocomposites and
their application to various organic reactions is presented herein. Focus is placed on various strategies for
achieving (1) Au nanoparticles (NPs) on Fe3O4@polymer catalysts, (2) optimized dispersion and stability
of Fe3O4/Pd catalysts, (3) Au NPs supported on Fe2O3–graphene oxide hybrid nanosheets and (4) rose-
like Pd–Fe3O4 hybrid nanocomposite-supported Au nanocatalysts. Further application of such hybrid
nanocomposites as catalysts for various organic reactions is discussed in brief.
Received 11 September 2015
Received in revised form
31 December 2015
Accepted 25 January 2016
Available online xxx
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Hybrid
Nanoparticles
Catalyst
Iron oxide
Noble metal
1. Introduction
development of hybrid nanocomposites to enable functionaliza-
tion of the surfaces of IONPs and develop new design strategies for
In recent years, numerous strategies have been attempted for
the design of hybrid nanostructures that combine the physical
and chemical properties of each component [1–9]. Many studies
have discussed the syntheses of such multicomponent nanostruc-
tures with increased functionality [10–20]. The multicomponent
functions combined with the enhanced chemical and physical
properties make the hybrid nanostructures suitable research tar-
gets for magnetic, plasmonic and semiconducting explorations
[21–25].
The design of iron oxide nanoparticles (IONPs) has gained partic-
ular attention because of fundamental scientific interest in various
cutting-edge technological applications of these species, including
their use as magnetic storage media, [26] in biosensing [27] and
medical applications [28,29] and as contrast agents in magnetic res-
onance imaging [30–33]. Thus, there has been a drive toward the
synthesis of IONPs with controlled size, composition and surface
properties in the subjects of extensive interdisciplinary research
[34,35]. It is well known that IONPs exhibit the tendency to clus-
ter, aggregate and lose their magnetic properties when applied
in solution and in biological systems. To enhance the stability of
these nanoparticles (NPs), intensive effort has been devoted to the
hybrid nanocomposites [36–42].
Hybrid nanocomposites are efficient catalysts for a wide range
of industrially relevant organic transformations. For example, pal-
ladium and gold NPs have attracted attention as catalysts for
carbon–carbon coupling reactions, reduction of 4-nitrophenol and
a series of other reactions [43–53]. Magnetic NPs or nanocom-
posites are considered as ideal supports because they efficiently
immobilize metal NPs and circumvent the complications of filtra-
tion (such as loss of catalyst, oxidation of sensitive metal complexes
and usage of additional solvents for precipitation steps).
In this review, we focus on recent developments in the synthe-
sis of hybrid iron oxide–noble metal nanocomposites and various
strategies for achieving (1) Au NPs on Fe3O4@polymer catalysts,
(2) optimized dispersion and stability of Fe3O4/Pd catalysts, (3) Au
NPs supported on Fe2O3–graphene oxide (GO) hybrid nanosheets
and (4) rose-like Pd–Fe3O4 hybrid nanocomposite-supported Au
nanocatalysts (Scheme 1). Further application of such hybrid
nanocomposites as catalysts to various organic reactions is also
discussed in brief.
2. Au NPs on Fe3O4@polymer catalysts [54]
Core–shell nanocomposites have recently emerged as promis-
ing catalysts, where the shell not only protects the core
from oxidation, but also facilitates surface modification and
∗
Corresponding author. Fax: +82 51 980 5200.
E-mail address: chemistry@pusan.ac.kr (K.H. Park).
http://dx.doi.org/10.1016/j.cattod.2016.01.030
0920-5861/© 2016 Elsevier B.V. All rights reserved.
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in the magnetic synthetic system, the synergetic hydrogen bonds
formed between the carboxyl groups and ester groups, as well as
between the carbonyl groups and hydroxyl groups, were strong
enough to synthesize P(EGDMA-co-MAA) on the magnetite surface.
Thus, hydrogen bond interactions played an important role during
the coating of the polymer onto the Fe3O4 microspheres [61,62].
X-ray diffraction (XRD) (Fig. 2a) was used to identify the crystal
phase of the Fe3O4 spherical aggregates and Fe3O4@P(EGDMA-
co-MAA)/Au. Specifically, the patterns could be assigned to the
(2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (5 3 3) reflections
of the cubic spinel structure of Fe3O4 (JCPDS No. 19-0629) and
the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) reflections of face centered
cubic (fcc) Au (JCPDS No. 04-0784). The elemental compositions
of the Fe3O4@P(EGDMA-co-MAA)/Au catalyst were obtained using
energy-dispersive X-ray spectroscopy (EDS) (Fig. 2c). The super-
conducting quantum interference device (SQUID) data (Fig. 2d)
shows the magnetic curves as a function of the applied field at
300 K. The saturation magnetization value of Fe3O4@P(EGDMA-co-
MAA)/Au was about 45.3 emu g−1, which was a similar to that of
Fe3O4@P(EGDMA-co-MAA) at about 46.4 emu g−1. There was no
drastic decrease of the magnetization after loading of the Au NPs.
Moreover, the remanence and coercivity of the Fe3O4 microspheres
were both close to zero, indicating superparamagnetism.
Scheme 1. Hybrid iron oxide–noble metal nanocomposites.
The UV/vis absorption of the reaction mixture was moni-
tored with time during the catalytic reduction of 4-nitrophenol
(Fig. 3). Notably, the absorption of 4-nitrophenol at 400 nm
decreased rapidly with a concomitant increase in the intensity
of the peak at 300 nm, which is attributed to the product of
the reduction, 4-aminophenol. In control experiments, only the
Fe3O4@P(EGDMA-co-MAA) microspheres were used as the cata-
lyst; no reaction was observed (entry 1, Table 1). In the absence
of NaBH4, the Fe3O4@P(EGDMA-co-MAA)/Au catalyst showed
no catalytic activity (entry 2, Table 1). As expected, increasing
the temperature enhanced the catalytic activity (entries 3 and
4, Table 1). In Fig. 3a, the reaction rate constant (k) is com-
pared under different temperatures with the use of 2.5 mol%
of Fe3O4@P(EGDMA-co-MAA)/Au and 100 equiv. of NaBH4. The
highest catalytic efficiency (0.330 min−1) was obtained at 35 ◦C.
The amount of catalyst as well as the amount of NaBH4 used
during the reduction was also optimized (entries 5 and 6, Table 1).
The reduction was completed in 45 s when Fe3O4@P(EGDMAco-
MAA)/Au was used as the catalyst (5.0 mol% of catalyst, 300 equiv.
of NaBH4 per equiv. substrate) (Fig. 3b). Fe3O4@P(EGDMA-co-
MAA)/Au microspheres with different loadings of Au (11 and
18 wt.%) were also used as catalysts, resulting in lower catalytic
activity (entries 7 and 8, Table 1). The Fe3O4@P(EGDMA-co-
MAA)/Au catalyst exhibited superior catalytic activity to that
previously reported for Au–CeO2 nanocomposites and hybrid Au
nanoparticle-GO nanosheets based on comparison of the turnover
frequency (TOF) value [67,68]. APTS-modified Fe3O4@P(EGDMA-
co-MAA)/Au and Fe3O4@SiO2/Au catalysts were synthesized
and applied in this reaction as catalysts (Fig. 4a and b). Au
NPs were immobilized on APTS-modified Fe3O4@P(EGDMA-
co-MAA) and Fe3O4@SiO2. However, catalysts with low metal
loadings were obtained as compared with the Fe3O4@P(EGDMA-
functionalization to overcome problems caused by oxidation and
aggregation of IONPs. These catalysts include metal oxides such as
SiO2, TiO2, and Al2O3, organic monolayers, and polymers [55–66].
We synthesized hybrid Au NPs on Fe3O4 microspheres, and then
coated these Fe3O4 microspheres with a polymer to facilitate load-
ing of the Au NPs and to prevent aggregation and oxidation of the
Fe3O4 microspheres (Scheme 2) [54].
Scheme 2 shows the total synthesis of the Fe3O4@polymer/Au
catalyst. Fe3O4 microspheres were synthesized using the solvother-
mal method [61,62]. This comprised the partial reduction of FeCl3
with ethylene glycol as a solvent, sodium acetate as a reducing
agent and trisodium citrate (Na3Cit) as an electrostatic stabi-
lizer at 200 ◦C. The resulting Fe3O4 microspheres are shown in
Fig. 1a, demonstrating their spherical morphology with a rough
surface and an average diameter of 142 nm (Fig. 2b). The Fe3O4
microspheres consisted of aggregates of small magnetite par-
ticles with a mean size of 3 nm, as observed by transmission
electron microscopy (TEM) (Fig. 1b). Au NPs with a diameter of
18 nm (Fig. 2b) were immobilized on the Fe3O4@polymer micro-
spheres through hydrogen bonding between the ester groups in
the polymer and the Au precursors (Fig. 1c and d). The poly-
merization was carried out by using the hydrophobic monomer
ethylene glycol dimethacrylate (EGDMA) as a cross-linker with
the hydrophilic monomer methacrylic acid (MAA) to increase the
hydrophilicity of the Fe3O4@polymer microspheres, thereby gen-
erating Fe3O4@P(EGDMA-co-MAA) core–shell microspheres. The
TEM image of the Fe3O4@P(EGDMA-co-MAA) microspheres in
Fig. 1b shows a well-defined core–shell structure, with a black
core and a gray shell, without the appearance of any secondary
polymer as shown in Fig. 1b. Because the Fe3O4 microspheres
had carboxyl groups on their surfaces due to the use of Na3Cit
co-MAA)/Au
catalyst.
The
Fe3O4@P(EGDMA-co-MAA)/Au
Scheme 2. An illustration of synthesis for Fe3O4@polymer/Au.
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Fig. 1. (a) Scanning electron microscopy (SEM) image of Fe3O4 microspheres. (b) TEM images and magnified image of Fe3O4@P(EGDMA-co-MAA). Scale bar shows 10 nm
(inset). (c) Fe3O4@P(EGDMA-co-MAA)/Au. (d) Magnified image of Fe3O4@P(EGDMA-co-MAA)/Au.
Table 1
Optimized reaction condition.
Entry
Catalyst (mol%)
NaBH4 (equiv.)
Time
TOF (h−1
)
1
2
3
4
5
6
7
8
9
10
Fe3O4@P(EGDMA-co-MAA) (2.5)
Fe3O4@P(EGDMA-co-MAA)/Au (2.5)
Fe3O4@P(EGDMA-co-MAA)/Au (2.5)
Fe3O4@P(EGDMA-co-MAA)/Au (2.5)
Fe3O4@P(EGDMA-co-MAA)/Au (2.5)
Fe3O4@P(EGDMA-co-MAA)/Au (5.0)
Fe3O4@P(EGDMA-co-MAA)/Au (5.0, Au base: 11 wt.%)
Fe3O4@P(EGDMA-co-MAA)/Au (5.0, Au base: 18 wt.%)
APTS-modified Fe3O4@P(EGDMA-co-MAA)/Au (2.5)
APTS-modified Fe3O4@SiO2/Au (2.5)
300
0
2 h
2 h
No reaction
No reaction
96
600
518
1600
1029
400
20
17.8
100
100
300
300
300
300
300
300
25 min
6 min
4 min 38 s
45 s
1 min 10 s
3 min
2 h
2 h 15 min
Reaction condition: 10 mL of 7.50 × 10−4 M 4-nitrophenol, 0.25 mg of Fe3O4@P(EGDMA-co-MAA)/Au [Au base (15 wt.%): 2.5 mol%], 1.0 mL of 2.22 M NaBH4 (300 equiv. to the
substrate), 25 ◦C.
catalyst showed higher catalytic efficiency (0.354 min−1) than
Fe3O4@P(EGDMA-co-MAA)/Au catalyst, uniform Au NPs were
immobilized onto Fe3O4@P(EGDMA-co-MAA) in situ without any
treatment of the functional groups, resulting in high catalytic
activity for reduction of 4-nitrophenol. Remarkably, after the
the APTS-modified Fe3O4@P(EGDMA-co-MAA)/Au (0.0127 min−1
)
and APTS-modified-Fe3O4@SiO2/Au (0.00993 min−1) (entries 5,
9 and 10, Table 1) congeners (Fig. 3c). In the synthesis of the
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Fig. 2. (a) XRD patterns of Fe3O4@P(EGDMA-co-MAA)/Au [a] and Fe3O4 microspheres [b], (b) Size distribution diagrams of the Au NPs and Fe3O4 microspheres, (c) EDS
spectrum of Fe3O4@P(EGDMA-co-MAA)/Au catalyst, (d) SQUID data.
reaction, the Fe3O4@P(EGDMA-co-MAA)/Au catalyst could be
extracted from the reaction mixture with an external magnet and
reused for four reaction cycles under the same reaction conditions
(Fig. 3d).
The resulting Fe3O4 microspheres are shown in Fig. 5. All of the
Fe3O4 microspheres were spherical with a rough surface. When
Na3Cit, polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP)
were used as capping agents, the respective average diameters of
the Fe3O4 microspheres were 142, 150, and 153 nm. However, the
average diameter increased to 175 nm in the absence of capping
agents (Fig. 5d). Each type of Fe3O4 microsphere consisted of aggre-
gates of small magnetite particles with a mean size of 3 nm, as
observed via TEM (Fig. 6).
3. Optimized dispersion and stability of Fe3O4/Pd catalysts
[69]
To fabricate a suitable hybrid magnetic catalyst, it is quite nec-
essary to develop techniques for controlling the dispersion of the
catalysts in various solvents to apply them to functional materials.
According to the classical Derjaguin–Landau–Verwey–Overbeek
(DLVO) theory, the stability of particle dispersion is determined
by the sum of the repulsive electrostatic forces and attractive van
der Waals forces [70]. Thus, several approaches have been utilized
for surface modification of NPs or microspheres to facilitate dis-
persion in organic solvents, and many organic capping agents have
been used for enhancing the dispersion stability and solubility of
IONPS [71–73]. Fe3O4 microspheres were synthesized by using dif-
ferent capping agents and the effect of the capping agents on the
dispersion stability and catalytic activity of the Fe3O4/Pd composite
for suzuki coupling reactions in water were evaluated (Scheme 3)
[69].
The TEM images show that Pd NPs with diameters of ca. 4 nm
were immobilized onto each Fe3O4 microsphere by substituting the
sodium cations with Pd ions in aqueous medium (Fig. 6) [61]. The
size distribution plots of each Pd NP are shown in Fig. 6, demon-
strating that the size of the Pd NPs was generally less than 5 nm.
During the catalyst preparation, because the Pd NPs were immobi-
lized onto Fe3O4 microspheres in aqueous solution, Na3Cit-Fe3O4
and PEG-Fe3O4 could be well dispersed in the aqueous solution due
to the hydrophilic functional groups of the capping agents, result-
ing in high monodispersity of the Pd NPs on the Fe3O4 microspheres
(Fig. 6a and b). The Fe3O4 microspheres without capping agents also
showed high dispersion in water because of the hydroxyl groups
present on magnetite, which resulted in strong immobilization of
the Pd NPs (Fig. 6c). The use of PVP as an organic capping agent
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Fig. 3. (a) Plot of ln(Ct/C0) versus time and the corresponding Arrhenius plot for the reduction of 4-nitrophenol over Fe3O4@P(EGDMA-co-MAA)/Au catalysts under different
temperatures at 2.5 mol% of catalyst and 100 equiv. of NaBH4. (b) Plot of ln(Ct/C0) versus time for the reduction of 4-nitrophenol with different catalysts. All catalysts are
used at the same molar ratio of 2.5 mol% of catalyst and 300 equiv. of NaBH4 for the reaction at R.T. [a] Fe3O4@P(EGDMA-co-MAA)/Au [b] APTS-modified Fe3O4@P(EGDMA-
co-MAA)/Au [c] APTS-modified-Fe3O4@SiO2/Au. Time-dependent UV/vis absorption spectra for the reduction of 4-nitrophenol over hybrid Fe3O4@P(EGDMA-co-MAA)/Au
catalyst in aqueous media at 298 K. (c) Fe3O4@P(EGDMA-co-MAA)/Au. (d) Recycle test.
Fig. 4. TEM images of Au NPs on (a) APTS-modified Fe3O4@polymer and (b) Fe3O4@SiO2 microspheres.
interrupted the dispersion of Fe3O4 in aqueous solution because of
its hydrophobic nature as compared to the other capping agents,
thereby making immobilization of the Pd NPs difficult (Fig. 6d).
XRD analysis (Fig. 7a) was used to identify the crystal phase of
the respective Fe3O4/Pd microspheres. Specifically, the peaks were
assigned to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), and (5 1 1) reflections
of the cubic spinel structure of Fe3O4 (JCPDS No. 19-0629) and the
(1 1 1), (2 0 0), and (2 2 0) reflections of fcc Pd (JCPDS No. 46-1043).
When the Fe3O4 microspheres were capped with Na3Cit, the over-
all intensity of the XRD peaks of Na3Cit-Fe3O4/Pd decreased. The
SQUID data presented in Fig. 7b show the magnetic curves as a
function of the applied field at 300 K. The saturation magnetiza-
tion value of Na3Cit-Fe3O4/Pd was 45.2 emu g−1, which is similar
to that of Na3Cit-Fe3O4 (ca. 48.2 emu g−1). The small decrease in
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Scheme 3. Dispersion-dependent catalytic activity of Fe3O4/Pd catalysts in water for suzuki coupling reactions.
Fig. 5. SEM images of (a) Na3Cit-Fe3O4, (b) PEG-Fe3O4, (c) No-Fe3O4 and (d) PVP-Fe3O4 microspheres.
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Fig. 6. TEM images of immobilized Pd NPs onto (a) Na3Cit-Fe3O4, (b) PEG-Fe3O4, (c) No-Fe3O4 and (d) PVP-Fe3O4 microspheres. Size distribution graphs of each Pd NPs.
the saturated magnetization value of the Na3Cit-Fe3O4/Pd micro-
spheres compared to that of the Na3Cit-Fe3O4 microspheres can be
attributed to the slight increase in mass due to the immobilized
Pd NPs on the surface of the Fe3O4 microspheres [74]. Moreover,
the remanence and coercivity of the Fe3O4 microspheres were both
close to zero, indicating superparamagnetism.
the catalytic activity of the hybrid catalysts; the results are sum-
marized in Table 2. These results suggest that the bases, reaction
time, and temperature utilized have dramatic effects on the yields
of the cross-coupling product in this reaction. At 80 ◦C, 91% con-
version was obtained in the mixture of DMF:H2O (4:1) (entry
1, Table 2). Interestingly, when the reaction temperature was
decreased to 50 ◦C using only H2O as a solvent, high catalytic activ-
ity was achieved with the Na3Cit-Fe3O4/Pd catalyst because this
As mentioned above, the suzuki coupling reactions between
bromobenzene and phenylboronic acid were used to investigate
Table 2
Optimized reaction conditions.
Entry
Cat. (mol%)
Temp. (◦C)
Time (h)
Base
Solvent
Conv. (%)a
1
2
3
4
5
6
7
8
1 (Na3Cit)
1 (Na3Cit)
80
50
50
50
50
50
40
40
40
100
100
25
50
50
50
5
5
5
5
7
5
K2CO3
K2CO3
K2CO3
Cs2CO3
Cs2CO3
CsOH
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
DMF:H2O (4:1)
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
91
97
65
89
98
66
48
94
76
80
98
13
89
57
37
0.1 (Na3Cit)
0.05 (Na3Cit)
0.05 (Na3Cit)
0.05 (Na3Cit)
0.05 (Na3Cit)
0.05 (Na3Cit)
0.1 (Na3Cit)
0.05 (Na3Cit)
0.05 (Na3Cit)
0.05 (Na3Cit)
0.05 (PEG)
12
24
12
1
1.5
24
7
9
10
11
12
13
14
15
0.05 (No)
0.05 (PVP)
7
7
Reaction conditions: Na3Cit-Fe3O4/Pd catalyst (Pd base: 0.05 mol%), bromobenzene (0.5 mmol), phenylboronic acid (0.6 mmol), Cs2CO3 (1.0 mmol), H2O (3.0 mL) and 50 ◦
for 7 h.
C
a
Determined by using GC-MS spectroscopy.
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Fig. 7. (a) XRD pattern of each Fe3O4/Pd catalysts, (b) SQUID data.
As expected, 98% conversion was achieved within 1.5 h at high
temperature (100 ◦C) (entries 10 and 11, Table 2). The conversion
decreased at room temperature, even though a reaction time of
24 h was used (entry 12, Table 2). The Na3Cit-Fe3O4/Pd catalyst
could be separated using an external magnet and could be recycled
five times under the same reaction conditions (entry 11, Table 2)
(Fig. 8).
Investigation of the dispersibility of nanomaterials in vari-
ous solvents is crucial for environment-friendly applications [75].
Changes in the surface charge of the nanostructure in liquid media
may produce drastic effects on the reactivity of nanomaterials
[76–78]. Thus, surface modification of catalysts is considered a
highly promising method for increasing the dispersion stability to
effectively enhance the catalytic activity without aggregation. Here,
we established a practical method for dispersing Fe3O4/Pd cata-
lysts in aqueous solution and confirmed the catalytic activity by
considering the differences in dispersibility. The stabilities of the
Fe3O4/Pd catalysts coated with different capping agents of various
hydrophilicities are shown in Fig. 9. Each Fe3O4/Pd catalyst was dis-
persed in water with a concentration of 1 mg/mL. The PVP-Fe3O4/Pd
microspheres began to form agglomerates within 30 min because of
the hydrophobicity of the PVP capping agent (Fig. 9a). No-Fe3O4/Pd
was more stable in water than PVP-Fe3O4/Pd due to the presence of
OH− anions on the surface of the bare Fe3O4 microspheres (Fig. 9b).
Agglomeration of the PEG-Fe3O4/Pd and Na3Cit-Fe3O4/Pd micro-
spheres began within 5 and 7 days, respectively (Fig. 9c and d).
Citrate and OH− anions on the Fe3O4 surface [61] enhanced the
Fig. 8. Conversion during five recycling runs.
catalyst, capped by Na3Cit, could be well dispersed in water (entry
2, Table 2). Preliminary screening for determining the most suitable
base system for the catalyst revealed that Cs2CO3 could efficiently
catalyze this reaction (entries 3, 4, and 6, Table 2). The optimum
reaction conditions were found to be as follows: catalyst: Na3Cit-
Fe3O4/Pd (0.05 mol%); solvent: H2O (3.0 mL), temperature: 50 ◦C,
reaction time: 7 h (entry 5, Table 2). At 40 ◦C, a reaction time of
24 h was necessary to obtain high yield (entries 7–9, Table 2).
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Fig. 9. Dispersion stability of Fe3O4/Pd catalysts after 30 min, 22 h, 5 days and 7 days, respectively. (PVP-Fe3O4/Pd, No-Fe3O4/Pd, PEG-Fe3O4/Pd and Na3Cit-Fe3O4/Pd).
Fig. 10. Zeta potential and reaction conversion of Fe3O4/Pd microspheres capped by different capping agents at pH 7. Reaction conditions: Fe3O4/Pd catalysts (Pd base:
0.05 mol%), bromobenzene (0.5 mmol), phenylboronic acid (0.6 mmol), Cs2CO3 (1.0 mmol), H2O (3.0 mL) and 50 ◦C for 7 h.
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Fig. 11. TEM images of (a, d)Au/Fe3O4 microspheres, (b, e) FeSO4·(H2O)-GO and (c, f) Au/Fe2O3-GO nanosheets.
Table 3
Surface areas, pore volumes and pore diameters of FeSO4·(H2O)-GO, Fe2O3-GO and Au/Fe2O3-GO.
Sample
BET surface area (m2 g−1
)
Langmuir surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Pore diameter (nm)
FeSO4(H2O)-GO
Fe2O3-GO
Au/Fe2O3-GO
18.62
213.0
212.4
25.75
291.8
292.3
0.0342
0.233
0.231
11.5
4.48
4.43
dispersion stability in water. The effect of the different capping
agents on the zeta potential at pH 7 is shown in Fig. 10 to con-
firm the dispersion stability in water. On the basis of the literature,
it was concluded that if the zeta potential of the particles is higher
than 30 mV or lower than −30 mV, the dispersion is stable [79].
As the zeta potential increased, the conversion during the suzuki
coupling reaction also increased (Fig. 10), indicating that the high
dispersion stability of the catalysts in water, which precludes aggre-
gation, leads to higher catalytic activity because of the increase in
the available catalytically active sites. The Na3Cit-Fe3O4/Pd cata-
lyst had the highest zeta potential (38.3 mV) and gave rise to the
greatest product conversion (98%) (Fig. 10). Because of the good
dispersion stability of the developed Na3Cit-Fe3O4/Pd catalyst in
water, this catalyst showed higher catalytic activity than previ-
ously reported Fe3O4-supported Pd nanocatalysts [80–82] and has
an environmentally positive impact and good overall industrial
applicability.
4. Au NPs supported on Fe2O3-GO hybrid nanosheets [83]
Noble metal NPs with metal oxide supports and graphene oxide
have been applied to organic reactions as catalysts [84–86]. For
Scheme 4. Synthetic scheme of Au/Fe2O3-GO hybrid nanosheets.
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instance, Deng et al. [85] developed multi-functional mesoporous
composite microspheres with well-designed nanostructures for an
integrated catalyst system. Scheuermann et al. [86] reported Pd NPs
on graphite oxide and their functionalized graphene derivatives
with the aim of creating a hybrid catalytic system. Iron oxide and GO
are important components used in a number of supports, since iron
oxide can be separated from the reaction mixture with an external
magnet and GO exhibits high electrical conductivity. Even though
iron oxide supports show high recyclability, they are unstable in
air and easily aggregate [87]. Moreover, although GO nanosheets
offer the advantage of having surface functional groups as well as a
large surface area [88], these nanosheets are not easily recycled by
using an external magnet; therefore, it is essential to combine the
advantages of iron oxide and GO. We synthesized FeSO4·(H2O)-GO
nanosheets by a one-pot method and subsequently immobilized Au
NPs onto this support [83]. During the in situ immobilization of the
Au NPs, FeSO4·(H2O) was transformed into Fe2O3 with consequent
generation of superparamagnetic Au/Fe2O3-GO nanosheets. To the
best of our knowledge, this is the first example of versatile synthesis
of Au/Fe2O3-GO nanosheets. Au/Fe2O3-GO nanosheets were then
used as a catalyst for the reduction of 4-nitrophenol in water, and
high catalytic activity and recyclability were achieved.
The Au/Fe2O3-GO nanosheets were synthesized from
FeCl3·6H2O via two steps (Scheme 4). FeSO4·(H2O)-GO nanosheets
were prepared by an in situ hydrothermal method [89]. A gold
precursor was subsequently injected to yield Au/Fe2O3-GO
nanosheets. GO nanosheets were used as the precursor to prepare
FeSO4·(H2O)-GO nanocomposites, which were synthesized from
natural graphite powders by a modified Hummer’s method [90].
For comparison, Au/Fe3O4 microspheres were also synthesized.
The TEM images show the morphology of the Au/Fe3O4 micro-
spheres, FeSO4·(H2O)-GO and Au/Fe2O3-GO nanosheets (Fig. 11).
No pretreatment procedures such as coating polymers or SiO2 onto
Fe3O4 were required prior to immobilization of the Au NPs onto
the Fe3O4 microspheres. In fact, the Au NPs were easily immobi-
lized on the surface of Fe3O4 in situ via interaction of the carboxyl
Fig. 12. XRD graphs of FeSO4·(H2O)-GO, Au/Fe2O3-GO and Au/Fe3O4.
groups of Na3Cit with Au (Fig. 11a and d) [61]. The XRD pattern
of the Au/Fe3O4 microspheres could be indexed to the cubic spinel
structure of Fe3O4 (JCPDS No. 19-0629) and the fcc structure of Au
(JCPDS No. 04-0784) (Fig. 12).
TEM images of the FeSO4·(H2O)-GO nanocomposites are shown
in Fig. 11b and e. The XRD pattern shown in Fig. 12 shows anorthic
FeSO4·(H2O)-GO reflections (JCPDS No. 81-0019). Elemental anal-
ysis of FeSO4·(H2O)-GO using X-ray photoelectron spectroscopy
(XPS) showed the Fe2+ 2p3/2 peak of FeSO4·(H2O) at 710.9 eV; the
binding energies of the doublet for S6+ 2p1/2 (168.1 eV) and S6+
2p3/2 (168.6 eV) are shown in Fig. 13 and are consistent with the
previously reported data for FeSO4·(H2O) (Fig. 13a and c) [91]. The
binding energy of the C–C peak is assigned as 284.3 eV and shifts
of +1.6 and +3.3 eV are typically assigned to the C OH and C
functional groups, respectively (Fig. 13b) [92].
O
Fig. 13. The XPS spectra of Fe2+, C and S6+ of FeSO4·(H2O)-GO (a–c) and Fe3+, C and Au of Au/Fe2O3-GO (d–f).
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Fig. 14. (a) Nitrogen-adsorption/desorption isotherms and (b) TGA data of FeSO4·(H2O)-GO, Fe2O3-GO and Au/Fe2O3-GO; for clarity, the isotherms of Fe2O3-GO and Au/Fe2O3-
GO were shifted upwards by 50 and 110 cm3 g−1, respectively.
In order to immobilize the Au NPs onto the FeSO4·(H2O)-GO
support, a method of exchanging sodium cations with noble metal-
lic ions was applied [61]. Iron salts precipitate in alkaline aqueous
solution [93], leading to the oxidation of FeSO4·(H2O) to Fe2O3
during immobilization of the Au NPs, resulting in formation of
Au/Fe2O3-GO nanosheets. The TEM images show highly monodis-
perse Au NPs immobilized on the Fe2O3-GO nanosheets (Fig. 11c
and f). Since it was difficult to assign the crystal phase of Fe2O3
due to the low intensity of the peaks in the XRD pattern (Fig. 12),
we confirmed the Fe2O3 structure by using the XPS technique. In
the case of high-spin iron, the Fe 2p peak is always split into two
due to spin-orbit coupling (Fe 2p1/2, Fe 2p3/2). In the oxidized state
(Fig. 13d), an additional satellite peak appears in-between the Fe
2p1/2 and Fe 2p3/2 components; in the case of Fe2O3, the satellite
occurs 8 eV above the peak of the Fe 2p3/2 component [94]. The
binding energy of C–C is also assigned as 284.3 eV and shifts of +1.5
and +3.8 eV are typically assigned to the C OH and C O functional
groups, respectively (Fig. 13e) [92].
GO was further oxidized when the Au/Fe2O3-GO nanosheets
were synthesized. The binding energies of the doublet for Au 4f7/2
(83.1 eV) and Au 4f5/2 (87.0 eV) shown in Fig. 13f are character-
istic of Au0 [95]. As a control experiment, Fe2O3-GO nanosheets
were also synthesized without addition of HAuCl4·3H2O. The
N2 adsorption/desorption isotherms (Fig. 14a) of Fe2O3-GO and
the Au/Fe2O3-GO nanosheets were typical IV isotherms, indicat-
ing the presence of mesopores (4.4 nm) and a high surface area
(292 m2 g−1). The textural characteristics of the corresponding
samples are summarized in Table 3. Surprisingly, the surface area
of FeSO4·(H2O)-GO was less than 20 m2 g−1 (Table 3). Thermogravi-
metric analysis (TGA) was also used to characterize the thermal
stability of the three samples (Fig. 14). The TGA data showed differ-
ent weight loss (%) trends for the FeSO4·(H2O)-GO and Fe2O3-GO
nanocomposites. It is postulated that the physical and chemical
Fig. 15. Time-dependent UV/vis absorption spectra for the reduction of 4-nitrophenol over hybrid catalysts in aqueous media at 298 K. (a) Au/Fe2O3-GO (5.0 mol%). (b)
Au/Fe3O4 (2.5 mol%). (c) Plot of ln(Ct/C0) versus time with different catalysts. All catalysts are used at the same molar ratio of 5.0 mol% of catalyst and 100 equiv. of NaBH4
for the reaction at R.T. [a] Au/Fe3O4 and [b] Au/Fe2O3-GO. (d) Plot of ln(Ct/C0) versus time and the corresponding Arrhenius plot over Au/Fe2O3-GO catalysts under different
temperatures at 5.0 mol% of catalyst and 50 equiv. of NaBH4.
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Fig. 16. (a) TOF during seven times recycling runs. Reaction condition: Au/Fe2O3-GO catalyst (Au base: 5.0 mol%), 200 equiv. of NaBH4 and 35 ◦C. (b) TOF during four times
recycling runs. Reaction condition: Au/Fe3O4 catalyst (Au base: 5.0 mol%), 200 equiv. of NaBH4 and 35 ◦C.
Fig. 17. SEM images of rose-like Pd-Fe3O4 (a, c) and Au/Pd-Fe3O4 (b, d) nanocomposites. The bars in the inset represent (c, d) 200 nm.
properties of FeSO4·(H2O)-GO changed drastically in alkaline solu-
tion during immobilization of the Au NPs.
better dispersion stability in water. Because Fe3O4 has many cit-
rate groups and OH− anions, the dispersion stability of Au/Fe3O4
in water is enhanced. As shown in Fig. 15d, the reaction rate
constant (k) was also compared with the use of different temper-
atures using 5.0 mol% of Au/Fe2O3-GO and 50 equiv. of NaBH4.
As expected, the highest catalytic efficiency (0.252 min−1) was
obtained at 35 ◦C. The Au/Fe3O4 and Au/Fe2O3-GO catalysts both
exhibited superior catalytic activity relative to previously reported
Au-Fe3O4 nanocomposites and Au-graphene nanosheets in terms
of the reaction rate constant (k) [96,97]. It is believed that the elec-
tronic structures of the Au and metal oxide components are both
modified by electron transfer across the interface of the Au-metal
As shown in Fig. 15a and b, the UV/vis spectrum of the reaction
mixture was monitored with time during the catalytic reduc-
tion of 4-nitrophenol. The reduction was complete in 12 min and
6 min when Au/Fe2O3-GO (5.0 mol%) and Au/Fe3O4 (2.5 mol%) were
respectively used as the catalysts (100 equiv. of NaBH4 per equiv.
substrate at 298 K) (Fig. 15a and b).
As shown in Fig. 15c, the reaction constant (k) of the Au/Fe3O4
catalyst was higher (0.284 min−1) than that achieved with the
Au/Fe2O3-GO (0.212 min−1) nanocomposites. The slightly higher
catalytic activity of the Au/Fe3O4 catalyst is attributed to its
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Fig. 18. TEM images of Pd–Fe3O4 (a, b) and Au/Pd–Fe3O4 (c, d). Au size distribution graph (d). HR-TEM images of Pd-Fe3O4 (e) and Au/Pd-Fe3O4 (f). The bars in the inset
represent (a, c) 20 nm.
Scheme 5. Synthetic scheme of Au/Pd–Fe3O4 nanocomposites.
oxide hybrid NPs, giving rise to oxygen vacancies on the interfa-
cial metal oxide that act as active sites for oxygen absorption and
activation [98,99]. Thus, the hybrid NPs are catalytically more active
than each individual component for the reduction of 4-nitrophenol.
Remarkably, after the reaction, the Au/Fe2O3-GO catalyst could
be recovered from the reaction mixture with an external magnet
and reused seven times with only slight loss of catalytic activity
(Fig. 16a). Even though the Au/Fe3O4 catalyst showed higher cat-
alytic activity than the Au/Fe2O3-GO catalyst, the reusability of the
Au/Fe3O4 catalyst declined drastically within four uses (Fig. 16b).
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5. Rose-like Pd–Fe3O4 hybrid nanocomposite supported Au
nanocatalysts [100]
Numerous strategies have been developed for the design of new
hybrid nanostructures with defined multicomponents by control-
ling the size and shape of these species through solution-growth
fabrication [101–103]. Among the developed species, various
multimetallic NPs have been reported by Mao et al. [104] and
Mazumder et al. [105]. Mao et al. [104] reported a simple chem-
ical synthetic route for producing FePt–Au hybrid nanostructures
as catalysts for oxygen reduction reactions. Mazumder et al. [105]
presented a unique approach for synthesizing core/shell-structured
Pd/FePt NPs. Recently, they reported the one-pot synthesis of
urchin-like FePd–Fe3O4 via controlled thermal decomposition of
Fe(CO)5 and reduction of Pd(acac)2 [106]. We reported the facile
synthesis of rose-like Pd–Fe3O4 nanocomposites, followed by
immobilization of Au NPs onto these Pd–Fe3O4 supports (Scheme 5)
[100]. These hybrid Au/Pd–Fe3O4 nanostructures showed high cat-
alytic activity for the tandem synthesis of 2-phenylindoles and
demonstrated magnetic recyclability.
Fig. 19. XRD patterns of Pd–Fe3O4 and Au/Pd–Fe3O4.
The rose-like Pd–Fe3O4 nanocomposites were synthesized by
facile decomposition of Fe(CO)5 and reduction of Pd(OAc)2 in
oleylamine and 1-octadecene. The products were analyzed by
inductively coupled plasma atomic emission spectroscopy (ICP-
AES). Under the described synthesis conditions, the as-synthesized
individual hybrid nanosheets, containing Pd and Fe3O4, formed
three-dimensional rose-like Pd–Fe3O4 nanocomposites. The Fe/Pd
ratios of the composites synthesized at 160 ◦C were controlled to
64:36–80:20 by increasing the amount of Fe(CO)5 from 0.15 to
0.45 mL.
Fig. 17 shows SEM images of the rose-like Pd–Fe3O4 and
Au/Pd–Fe3O4 hybrid nanocomposites with a well-distributed aver-
age diameter of 213 nm (Pd–Fe3O4). Au NPs were uniformly
immobilized onto the Pd-Fe3O4 supports (Fig. 17b and d). The TEM
images show Pd–Fe3O4 nanocomposites with an overall Fe/Pd ratio
of 64:36 (Fig. 18a and b). The Au NPs immobilized on the Pd–Fe3O4
nanocomposites are shown in Fig. 18c and d. The immobilized Au
NPs, with an average diameter of 5.8 nm, were spherical and highly
monodisperse (Fig. 18d). The HR-TEM image of a representative
Pd–Fe3O4 nanocomposite (Fig. 18e) shows that the Pd and Fe3O4
structures have well-defined and uniformly spaced lattice fringes
with a measured fringe distance of 0.22 nm for the Pd (1 1 1) plane
and 0.30 nm for the Fe3O4 (2 2 0) plane, corresponding to the fcc
Pd and Fe3O4 structures, respectively. The Au NPs also have uni-
formly spaced lattice fringes, with a measured fringe distance of
0.24 nm, corresponding to the (1 1 1) interplanar spacing in the fcc
Au structure (Fig. 18f).
The 3D flower-like structures are highly advantageous for use in
sensors, lithium ion batteries, and photocatalysis because of their
large specific surface area [107]. Brunauer–Emmett–Teller (BET)
surface area analysis showed the surface area of the Pd–Fe3O4
nanocomposites to be 23.79 m2 g−1, which is much higher than
that of the previously reported hollow and solid sphere-like Fe3O4
microspheres (12.27 and 5.43 m2 g−1, respectively) because of the
individual Pd–Fe3O4 nanosheets [108].
The structures of the Au/Pd–Fe3O4 and Pd–Fe3O4 composites
were further characterized by XRD. Fig. 19 shows the XRD pat-
terns of the as-synthesized composites with overall element ratios
of 7:77:16 (Au:Fe:Pd) and 64:36 (Fe:Pd). In the case of the Pd–Fe3O4
structure, all the peaks of the XRD pattern could be assigned to the
(1 1 1), (2 0 0), and (2 2 0) lattice planes of the fcc Pd crystal structure
(JPCDS No. 46-1043) and to the (2 2 0), (3 1 1), (4 2 2), (5 1 1), and
(4 4 0) lattice planes of the cubic spinel structured Fe3O4 (JCPDS No.
19-0629). In the case of Au/Pd–Fe3O4, the reflections of the immo-
bilized Au NPs were assigned to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1)
planes of the fcc Au crystal structure (JCPDS No. 04-0784). Specifi-
cally, the broadened diffraction peaks of the Au NPs are indicative
of the small size of the crystal domains [109]. High-angle annu-
lar dark-field scanning TEM (HAADF-STEM) imaging and elemental
Fig. 20. HAADF-STEM image and elemental mapping of Au/Pd–Fe3O4. The bars in each element image represent 50 nm.
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Table 4
Tandem synthesis of 2-phenylindoles.
Entry
Catalyst
Temp. (◦C)
Time (h)
Base
Conversion (%)a
1
2
3
4
5
6
7
8
9
Pd–Fe3O4
Pd–Fe3O4
Pd–Fe3O4
Pd–Fe3O4
120
120
120
120
120
120
150
150
150
150
18
18
18
18
18
18
18
9
Piperidine
Piperidine
Piperidine
LiOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
CsOAc
Traceb
3b
41
45
48
57
97
97
Pd–Fe3O4
Au/Pd–Fe3O4
Au/Pd–Fe3O4
Au/Pd–Fe3O4
Au/Pd–Fe3O4
Au/Pd–Fe3O4
6
9
59
10
38c
Reaction condition: Au/Pd–Fe3O4 catalyst [Au base: 0.18 mol%, Pd base: 0.5 mol%], 2-iodoaniline (0.5 mmol), phenylacetylene (0.6 mmol), base (1.0 mmol), DMSO (2.5 mL).
a
Determined by using GC-MS spectroscopy based on 2-iodoaniline.
DMF and DMA was used as solvent, respectively.
0.09 mol% (Au base) of catalyst was used.
b
c
mapping also reveal that Au, Pd, and Fe3O4 are dispersed over the
entire area of the composite, confirming the hybrid Au/Pd–Fe3O4
structure (Fig. 20).
To evaluate the catalytic activity of the Au/Pd–Fe3O4 catalyst,
tandem synthesis of 2-phenylindoles from 2-iodoanilines with
phenylacetylenes was performed as a model reaction under dif-
ferent conditions (Table 4). First, the effects of the solvents were
examined in the presence of 0.5 mol% Pd–Fe3O4 catalyst at 120 ◦C
using a reaction time of 18 h (entries 1–3, Table 4). The use of more
polar solvents (DMSO) led to an increase in the conversion because
of the superior solubility of the reactant and catalyst in the reac-
tion medium. The influence of bases such as piperidine, LiOAc, and
CsOAc was studied for the standard reaction (entries 3–5, Table 4).
The use of CsOAc led to good conversion (48%) compared to
that achieved with piperidine and LiOAc. It was assumed accord-
ing to the hard-soft acid and base theory that Cs+ is the best
Pearson acid for removal of iodide from the transient palladium
NPs by maximization of the soft-soft interaction [110]. To date,
dual-catalytic cross-coupling reactions with gold and palladium
have continued to receive attention because of the novel reactiv-
Fig. 21. Comparison of catalytic activity with previous reported heterogeneous cat-
alysts in our group. Reaction condition: 2-iodoaniline (0.5 mmol), phenylacetylene
(0.6 mmol), CsOAc (1.0 mmol), DMSO (2.5 mL), Catalyst (0.36 mol%), 150 ◦C, 4.5 h.
ity that is unavailable in single-metal systems [111–113]. These
dual-catalytic systems possess higher catalytic activities than sin-
gle catalytic systems due to electron transfer across the interface
and circumvent the formation of the stochiometric transmeta-
lation byproducts that are common to cross-coupling reactions.
Thus, the Au/Pd–Fe3O4 catalyst showed better catalytic activity
than Pd–Fe3O4 since the Au NPs are highly efficient for activat-
ing phenylacetylene (entries 5 and 6, Table 4) [112]. As expected,
97% conversion was achieved at high temperature (150 ◦C) (entries
6 and 7, Table 4). The effects of the catalyst loading and reac-
tion time were also evaluated. The conversion decreased (59 and
38%) when a short reaction time (6 h) and a lower catalyst load-
ing (Au base: 0.09 mol%) were employed (entries 8–10, Table 4).
The optimum reaction conditions were found to be as follows:
Au/Pd–Fe3O4 (Au base: 0.18 mol%, Pd base: 0.5 mol%); solvent:
DMSO (2.5 mL), temperature: 150 ◦C, reaction time: 9 h (entry 8,
Table 4). Without the use of additives and ligands, the Au/Pd–Fe3O4
catalyst exhibited superior catalytic activity relative to previously
reported copper and gold complexes and Au NPs in terms of the
TOF value [114–116]. Catalytic activity was also compared with
previous reported heterogeneous catalysts of our group (Fig. 21)
[54,69,117–119]. Among them, Au/Pd–Fe3O4 exhibited the high-
est TOF value at the same condition and this could be attributed
to dual-catalytic system between catalytic effective Pd–Fe3O4 sup-
port and Au NPs. The Pd–Fe3O4 support can also exhibit high
catalytic activity since the individual Pd–Fe3O4 nanosheets with
high surface area possess Pd◦ sites that can act as catalytically active
sites. After the tandem reaction, the Au/Pd–Fe3O4 catalyst could
be totally separated from the reaction medium with an external
magnet owing to the superparamagnetism of the Fe3O4 particles
[120]. The Au/Pd–Fe3O4 catalyst was recycled three times and its
initial high activity (>99%) was still maintained without any loss
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ICP-AES analysis of the filtered solution after the catalytic reaction,
demonstrating a low content of Pd and Au of 79.8 and 80.9 ppm,
respectively, in the solution.
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The synthesis of hybrid iron oxide–noble metal nanocom-
posites was reviewed herein with focus on Au/Fe3O4@polymer,
Fe3O4/Pd, Au/Fe2O3-GO, and Au/Pd–Fe3O4 composites. These
hybrid nanocomposites act as efficient catalysts for reduction of
4-nitrophenol and in the suzuki coupling reaction and tandem
synthesis of 2-phenylindoles. All of the hybrid catalysts exhibited
better catalytic activity and recyclability than many previously cat-
alysts. Hybrid noble metal NPs with supports are demonstrated
to have great potential for tailoring of the activity, selectivity, and
stability by judicious selection and exploitation of their numerous
combinations.
Acknowledgments
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This research was supported by Basic Science Research Pro-
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funded by the Ministry of Science, ICT
& Future Planning
(NRF-2015R1D1A1A02060684 and NRF-2013R1A1A2012960) and
GCRC-SOP (No.2011-0030013). K.H.P thanks to the TJ Park Junior
Faculty Fellowship and LG Yonam Foundation.
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