Facile formation of gold nanoparticles on periodic mesoporous bipyridine-silica

Article abstract of DOI:10.1016/j.cattod.2017.03.012

Silica-supported gold nanoparticles (AuNPs) can be synthesized on a bipyridine incorporated periodic mesoporous organosilica (BPy-PMO) as an inorganic support. Reaction of a bipyridine group in the periodic mesoporous organosilica with HAuCl₄ f

Full text of DOI:10.1016/j.cattod.2017.03.012

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Catalysis Today
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Facile formation of gold nanoparticles on periodic mesoporous
bipyridine-silica
Nobuhiro Ishitoa,b, Kiyotaka Nakajimaa,b, Yoshifumi Maegawac,d, Shinji Inagakic,d
,
Atsushi Fukuokaa,b,∗
a Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido 001-0021, Japan
b Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan
c Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan
d Japan Science and Technology Agency (JST)/ACT-C, Nagakute, Aichi 480-1192, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Silica-supported gold nanoparticles (AuNPs) can be synthesized on a bipyridine incorporated periodic
mesoporous organosilica (BPy-PMO) as an inorganic support. Reaction of a bipyridine group in the
periodic mesoporous organosilica with HAuCl4 forms an AuCl2-based complex, and its structure cor-
responds to a homogeneous complex, [AuCl2(bpy)]Cl. Thermal reduction of the complex in H2 results in
the formation of small gold nanoparticles with an average size of ca. 3.8 nm. Combined with nitrogen
adsorption, XRD, UV/Vis, TEM, and XAFS measurements, it is demonstrated that uniform gold nanoparti-
cles are homogeneously distributed inside mesopores. The size and distribution of gold nanoparticles are
mainly controlled by strong interaction of surface metallic Au with pore walls of the periodic mesoporous
organosilica. AuNPs/BPy-PMO shows higher catalytic activity than Au/MCM-41 in aerobic oxidation of
benzaldehyde.
Received 10 October 2016
Received in revised form 22 January 2017
Accepted 6 March 2017
Available online xxx
Keywords:
Gold nanoparticles
Metal complex
Periodic mesoporous organosilica
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
of methods were reported for the preparation of supported AuNPs
catalysts such as co-precipitation [11], photochemical synthesis
Gold nanoparticles (AuNPs) have been widely studied as a
promising material for catalysis, electronic and photonic device,
and drug delivery system [1–7]. Despite bulk Au metal has no
functionality for catalysis, gold particles in nanometer-scale show
very unique activity for a variety of reactions, which is represented
by low temperature oxidation of CO with O2 [8,9]. Because cat-
alytic performance of AuNPs strongly depends on their particle size,
Haruta et al. developed the deposition precipitation method to form
highly dispersed AuNPs on various metal oxide supports [8,10]. This
method is solely based on impregnation of preformed Au(OH)3 with
inorganic supports, and it is able to control the size of resultant
AuNPs by optimization of pH in solution containing HAuCl3 pre-
cursor. In contrast, direct impregnation of ionic gold precursors on
inorganic supports and subsequent reduction to form AuNPs is not
simply accomplished by a conventional combination of evapora-
tion and reduction technique, owing to low affinity of Au with the
supports resulting in the formation of large Au particles. A variety
[12], solid-solid reactions of a gold complex with the supports [13],
liquid-phase reduction [14,15], and impregnation of AuNPs sta-
bilized with sulfur or phosphorus-containing organic compounds
and polymers [16–18]. Fe2O3 and TiO2 are useful supports for the
preparation of well-defined AuNPs; however, silicas with high sur-
face area, which are widely used as supports for organic functional
groups and novel metals, are not applicable as host materials due
to difficulties in controlling the size below 6 nm.
Periodic mesoporous organosilicas (PMOs) are unique meso-
porous materials that consist of crystal-like ordered arrays of
organic moieties bridged by siloxane bonds [19,20]. Introduction
of organic groups into the PMOs can lead to various chemical and
physical properties [21–23]. Recently, a PMO with 2,2ꢀ-bipyridyl
(BPy) groups has been synthesized from a bridged precursor,
5,5ꢀ-bis(triisopropoxysilyl)-2,2ꢀ-bipyridine, in the presence of a
structure-directing agent. 2,2ꢀ-Bipyridyl group incorporated into
siloxane network is a versatile ligand and readily available as a
chelating unit for the formation of Ir, Ru, Re, and Pd complexes
[24–26]. Herein, we report a novel strategy for the immobiliza-
tion of AuNPs on BPy-PMO through the formation of Au complex
with surface BPy moiety and subsequent thermal reduction. First,
an Au complex was formed by the treatment of BPy-PMO with
∗
Corresponding author at: Institute for Catalysis, Hokkaido University, Sapporo,
Hokkaido 001-0021, Japan.
E-mail address: fukuoka@cat.hokudai.ac.jp (A. Fukuoka).
http://dx.doi.org/10.1016/j.cattod.2017.03.012
0920-5861/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: N. Ishito, et al., Facile formation of gold nanoparticles on periodic mesoporous bipyridine-silica, Catal.
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Au L3-edge XAFS measurement, which provides information on
the electronic state and structure of Au species stabilized on the
support. In the X-ray absorption near-edge structure (XANES) in
Fig. 1a, the edge energy of Au species on AuCl3-BPy-PMO appeared
at 11,917 eV which corresponds to a trivalent Au species. Because
AuCl3-BPy-PMO and [AuCl2(bpy)]Cl show almost the same spec-
tra, electronic structure of Au atoms in AuCl3-BPy-PMO is the same
as that of trivalent Au species in [AuCl2(bpy)]Cl. This trend was
also observed for extended X-ray absorption fine structure (EXAFS)
oscillation (Fig. 1b) and Fourier transforms of EXAFS (Fig. 1c);
AuCl3-BPy-PMO gave almost the same spectrum as [AuCl2(bpy)]Cl.
These results show that a similar structure to that of the corre-
sponding homogeneous complex, [AuCl2(bpy)]Cl, is successfully
formed on the surface of BPy-PMO via a BPy group as a chelating
unit as shown in Fig. 1d.
Scheme 1. Formation of Au complex on BPy-PMO.
HAuCl4·4H2O in ethanol. These complexes are easily transformed
to AuNPs by a simple reduction with thermal treatment of the Au
complex-stabilized BPy-PMO with H2. Results of characterization
and catalysis of AuNPs/BPy-PMO are also presented.
2. Experimental
2.1. Reagents
HAuCl4·4H2O was purchased from Wako Pure Chemical Indus-
tries, Ltd. Other reagents were obtained with the highest grade and
used without further purification.
3.2. Formation of Au nanoparticles on BPy-PMO
We performed thermal reduction of AuCl3-BPy-PMO at 423 K
in H2 to obtain BPy-PMO-supported AuNPs. Fig. 2 shows N2
adsorption-desorption isotherms and non-linear DFT (NLDFT) pore
size distribution curves of bare BPy-PMO and AuNPs/BPy-PMO.
Both bare BPy-PMO and AuNPs/BPy-PMO have type-IV isotherms
with no pronounced hysteresis loops, and this result is typical of
mesoporous materials with small-sized mesopores (Fig. 2a) [28].
NLDFT calculation with the isotherms provides pore size distri-
bution curves of these samples, which reveals the presence of
uniform mesopores of ca. 4.4 nm in diameter (Fig. 2b). While
Brunauer–Emmett–Teller (BET) surface area slightly decreased
from 670 to 620 m2 g−1, it is apparent that original mesoporosity
of BPy-PMO retained intact after the formation of Au complex and
subsequent H2 reduction. Fig. 3 shows XRD patterns of bare BPy-
PMO and AuNPs/BPy-PMO in (a) low and (b) high-angle regions.
There is one intense diffraction at 2␪ = 1.69◦ that can be assigned
to (100) plane of p6 mm symmetry for mesoporous arrangement.
Despite the presence of some diffractions due to periodic structure
of BPy units in high-angle region, there are no signals of Au metal,
suggesting the formation of small nanoparticles undetectable by
XRD measurement. The amount of Au loaded on BPy-PMO was
estimated to be 1.1 wt% by EDX analysis.
As a control experiment, MCM-41-supported Au particle with
the same Au loading was prepared by an impregnation method.
Bare and Au-loaded MCM-41 s were also characterized by N2
adsorption measurement and XRD (Fig. 4). There is no significant
difference in N2 adsorption-desorption isotherms (Fig. 4a); BET sur-
face area and NLDFT pore diameter of bare MCM-41 are 760 m2 g−1
and 4.5 nm, and those of Au-loaded MCM-41 are 750 m2 g−1 and
4.3 nm. In contrast, new sharp diffractions are clearly observed in
high-angle region of the XRD pattern after gold metal formation
(Fig. 4b). They are assigned to (111), (200), (220), (311), and (222)
planes on the basis of the fcc structure of gold metal. The crystallite
size was determined by the Debye-Scherrer’s equation for the (111)
diffraction to be ca. 38 nm. Aggregation of small AuNPs formed
inside mesopores preferably occurred to form large-sized AuNPs on
the outer surface of MCM-41 particles even though Au cations were
homogeneously immobilized on the silica surface before thermal
reduction. This phenomenon can be induced by weak interaction
of AuNPs with silica surface. It should be noted that these large Au
particles are not formed on BPy-PMO.
2.2. Synthesis of BPy-PMO supported AuNPs
BPy-PMO was synthesized using 5,5ꢀ-bis(triisopropoxysilyl)-
2,2ꢀ-bipyridine as a bridged silica precursor and octadecyltrimethy-
lammonium chloride as
a surfactant [24]. An Au complex
immobilized BPy-PMO, denoted AuCl3-BPy-PMO, was prepared
from BPy-PMO and HAuCl4·4H2O as follows: a 100 mg of BPy-PMO
was added to a mixture of EtOH (30 mL) and HAuCl4·4H2O (100 mg,
0.50 mmol) under an Ar atmosphere. After stirring the mixture
under reflux condition for 6 h, the resulting solid was filtered,
washed with excess amounts of EtOH, and dried at 298 K in vacuo
to afford AuCl3-BPy-PMO (110 mg) as a pale yellow powder. The
solid was conducted with thermal reduction in H2 at 423 K for 1 h
to give BPy-PMO-supported AuNPs which is denoted AuNPs/BPy-
PMO. For comparison, an MCM-41-supported AuNPs was prepared
by a conventional impregnation method using HAuCl4·4H2O. After
immobilization of Au species on MCM-41 by vacuum evaporation
at 298 K, the dried sample was also reduced in H2 at 423 K for 1 h to
obtain MCM-41-supported AuNPs (Au/MCM-41). A model complex
for AuCl3-BPy-PMO, [AuCl2(bpy)]Cl, was synthesized according to
a procedure in the literature [27] and purified by recrystallization.
The samples were characterized using UV/Vis diffuse reflectance
spectroscopy (UV/Vis DRS; Jasco, V-650), X-ray fluorescence
spectroscopy (XRF, Shimadzu EDX-720), nitrogen adsorption
(MicrotracBEL Belsorp-mini II), X-ray diffraction (XRD, Rigaku
Ultima IV, Cu K␣), transmission electron microscopy (TEM, JEOL
JEM-2100F, 200 kV), and X-ray absorption fine structure (XAFS,
BL14B2 of SPring-8).
2.3. Oxidation of benzaldehyde with supported AuNPs
A mixture of benzaldehyde (11 mg, 0.1 mmol), catalyst (10 mg),
NaHCO3 (17 mg, 0.2 mmol), and distilled water (5.0 mL) was placed
into a Pyrex vial (20 mL) and stirred at 303 K for 2 h with bubbling
of oxygen gas at a flow rate of 10 mL min−1. After the reaction, the
mixture was analyzed using gas chromatography (GC; Shimadzu
GC-14B, flame ionization detector) with an HR-20 m (0.25 mm
diameter, 30 m long) column. Naphthalene was used as an internal
standard in this experiment.
The formation of AuNPs on BPy-PMO was further evaluated in
more details by UV/Vis DRS and TEM measurements (Fig. 5). A plas-
mon absorption peak derived from AuNPs appeared around 530 nm
in AuNPs/BPy-PMO (Fig. 5a). TEM image of AuNPs/BPy-PMO gives
direct information for AuNPs formation with an average size of
3.8 nm (Fig. 5b) and their homogeneous distribution inside and out-
side of mesopores whereas some large AuNPs in size of 6–8 nm are
3. Results and discussion
3.1. Synthesis of Au complex on BPy-PMO
The formation of gold complexes by thermal treatment of BPy-
PMO with HAuCl4·4H2O in ethanol (Scheme 1) was confirmed by
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Fig. 1. (a) Au L3-edge XANES, (b) EXAFS oscillation, (c) Fourier transforms of EXAFS spectra for AuCl3-BPy-PMO (red line) and [AuCl2(bpy)]Cl (black line), and (d) proposed
structure of AuCl3-BPy-PMO. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) N2 adsorption isotherms and (b) NLDFT pore diameter distribution curves of BPy-PMO (black line) and AuNPs/BPy-PMO (blue line). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this article.)
simultaneously observed on the external surface in Fig. 5b. AuNPs
formed inside mesopores of BPy-PMO are highly stabilized with
strong interaction between metallic Au and pore wall of BPy-PMO,
which restricts formation of large-sized AuNPs typically observed
for MCM-41 as shown above. In addition, mesopores effectively
prevent growing the size of AuNPs below the poresize (4.4 nm).
Thus, immobilization of AuNPs via the formation of gold complexes
and subsequent H2 reduction is definitely a simple and promising
method to synthesize supported AuNPs on BPy-based PMO support
in comparison with a conventional impregnation method.
Au L3-edge XAFS measurement was conducted for the evalua-
tion of Au species of AuNPs/BPy-PMO. Fig. 6a and b show XANES
and Fourier transforms of EXAFS spectra of AuNPs/BPy-PMO and Au
foil. These samples give similar edge energy (11,919 eV), and EXAFS
spectrum of AuNPs/BPy-PMO bears a great resemblance to that of
Au foil, indicating complete reduction of Au complex on BPy-PMO
and simultaneous formation of AuNPs. Weak and broad signals for
metallic Au and the absence of small signals in range from 4 to 7 Å
can be confirmed in the EXAFS spectrum of AuNPs/BPy-PMO. These
features are characteristic of small AuNPs [29,30].
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Fig. 3. XRD patterns of BPy-PMO (black line) and AuNPs/BPy-PMO (blue line) in (a) low and (b) high-angle region. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
Fig. 4. (a) N2 adsorption isotherms and (b) XRD patterns of bare (black line) and Au-loaded MCM-41 (blue line). (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
Fig. 5. (a) UV/Vis DRS spectra of bare and AuNPs/BPy-PMO and (b) TEM image of AuNPs/BPy-PMO.
Finally, catalytic activity of the supported gold catalysts was
examined using a test reaction, aerobic oxidation of benzaldehyde
in water at 303 K (Fig. 7). Au/MCM-41 oxidized benzaldehyde to
benzoic acid, giving 39% conversion and 22% yield toward ben-
zoic acid. With the same Au loading, AuNPs/BPy-PMO afforded 91%
conversion and 87% yield under the same conditions; yield of ben-
zoic acid for AuNPs/BPy-PMO was approximately four times that
for Au/MCM-41. Such a high activity can be ascribed to small Au
nanoparticle on BPy-PMOs, which is frequently observed for cat-
alytic reactions by supported Au nanoparticles [5,8]. The formation
of small Au nanoparticles on BPy-PMO is potentially an effective
strategy for the development of a highly active heterogeneous cat-
alyst for various aerobic oxidation reactions.
4. Conclusions
Complex formation and subsequent reduction are an effective
method for the formation of small AuNPs with homogeneous dis-
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Fig. 6. (a) Au L3-edge XANES and (b) Fourier transforms of EXAFS spectra for AuNPs/BPy-PMO (blue line) and Au foil (black line). (For interpretation of the references to color
in this figure legend, the reader is referred to the web version of this article.)
Fig. 7. Aerobic oxidation of benzaldehyde in water with supported Au catalysts.
tribution on 2,2ꢀ-bipyridine-based PMO material. Average diameter
of AuNPs is estimated to be ca. 3.8 nm, which is much smaller than
those formed on silica supports reported in the previous literatures.
Mesoporous structure is one important factor controlling the size
of AuNPs smaller than that of mesopores. AuNPs/BPy-PMO shows
higher catalytic activity than Au/MCM-41 in oxidation of benzalde-
hyde. We believe that this methodology is useful to synthesize
silica-supported AuNPs.
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