One-pot fabrication of yolk-shell nanospheres with ultra-small Au nanoparticles for catalysis

Article abstract of DOI:10.1039/c4cc10237a

We report a "one-pot" method for the direct synthesis of an organosilica shell/silica core nanoreactor confined with ultra-small metal (Au, Pd, and Ru) nanoparticles. The nano-reactor confined with Au nanoparticles showed high activity towards styrene oxi

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One-pot fabrication of yolk-shell nanospheres with
ultra-small Au nanoparticles for catalysis
Cite this: DOI: 10.1039/x0xx00000x
ab
ab
a
Yi Yao,ab Xiaoming Zhang, Juan Peng and Qihua Yang *
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a “one-pot” method for the direct synthesis of layer.11 However, extra layer coating of metal NPs may cause the mass
organosilica-shell/silica core nanoreactor confined with ultra- transfer barrier during the catalytic process.
small metal (Au, Pd, Ru) nanoparticles. The nano-reactor Another alternative method is the encapsulation of metal NPs in core-
confined with Au nanoparticles showed high activity towards shell or the yolk-shell nanostructures.12 For example, Shi and co-
styrene oxidation using O2 as oxidant under 1 atm pressure authors fabricated a magnetic core/mesoporous silica shell nano-
and could be stably recycled without deterioration on both structure to fulfill targeted drug delivery and multiphase separation;13
conversion and selectivity. The strategy could be adapted Zhang synthesized yolk-shell microspheres containing a single Au
onto other nanostructures with little modification to obtain nanoparticle core and a mesoporous silica shell and its catalytic
yolk-shell nanoreactors for catalysis application.
activity was tested in the reduction of 4-nitrophenol with NaBH4.14 For
core-shell nanostructures, metal NPs cores are tightly surrounded by
the core material,15 while there is enough space for metal NPs in the
yolk-shell nanostructures. Yolk-shell nanospheres is more suitable for
low temperature liquid phase reactions because the hollow interiors
could provide enough space for the catalytic reactions and weak
interaction between metal NPs and yolk-shell nanospheres may help
to keep the intrinsic properties of metal NPs. Compared with core-
shell nano-spheres, the synthesis of yolk-shell nanospheres is more
challenging especially for encapsulating ultra-small metal NPs, even if
methods such as ship-in-bottle approach, soft templating approaches,
template-free approaches, Ostwald ripening or galvanic replacement
processes, and methods based on the Kirkendall effect, have been
developed. Moreover, tedious multi-step synthesis is generally
involved.16
Precious metal catalysts are widely used in modern chemical industry
in fields such as hydrogenation,1 oxidation,2 refining process3 and etc.
Since the pioneering work of Haruta who found that CO could be
oxidized on supported Au NPs even at a sub-ambient temperature,4
Au NPs have been widely employed in the oxidation of hydrocarbons,5
epoxidation of olefines,6 hydrogenation reactions7 and so on.8 The
melting point of gold decrease rapidly when particle size was reduced
into nanoscale, resulting in the easy aggregation of highly active
species.9 In most cases, precious metal nanoparticles (NPs) were
loaded onto supports with high surface area to pursue the utmost
dispersion and thus to expose active sites to the most extent.
However, the growth of particle-size under high temperature or
during the reaction process generally results in the deterioration of
the catalytic activity. This phenomena could always be observed
especially in cases that the metal NPs are of ultra-small size (<10 nm)
or have a low melting point, which presents one of the most common
and serious problems in precious metal catalysis.
CTAB
Ammonia
Au-PVP
BTME
nano-particle
Several methods were developed to prevent the aggregation of metal
NPs. Recently, it was reported that stability of metal NPs could be
greatly enhanced by coating metal NPs with either oxide layer or
carbon layer. For example, Xie tried to cut off the fusion of individual
Au NPs by coating pre-prepared Au NPs with SnO2.10 Atomic layer
deposition was used by Peter and co-authors for preparing a coking-
and sintering-resistance catalyst by covering Pd NPs with an Al2O3
TEOS
SiO2 Core
Au@SiO2 Core-Shell
Au@O-SiO2 York-Shell
Scheme 1. Schematic illustration for the preparation of Au@O-SiO2 Y-S
nanoreactors
In this work, we reported a general one-pot method for the synthesis
of yolk-shell nano-spheres encapsulated with ultra-small Au NPs using
PVP protected Au (Au-PVP) as metal NPs source and TEOS
(tetraethoxysilane) and BTME (1,2-bis(trimethoxysilyl)ethane) as silica
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source in base medium in the presence of CTAB (hexadecyl trimethyl confirms the adherence of Au-PVP on the surface of silica core after
ammonium bromide). It was found that Au-PVP confined in yolk-shell its addition to the synthesis medium.
nanospheres is highly active in liquid-phase styrene oxidation with
oxygen as oxidant. The catalyst could be facilely recycled for 8 times
without obvious deterioration in activity. Compared with catalysts
prepared by conventional impregnation method, Au NPs in yolk-shell
nanospheres showed excellent resistance to the aggregation during
the catalytic process.
The general process for the synthesis of yolk-shell nano-spheres
encapsulated with Au NPs involves the hydrolysis of TEOS in basic
water-ethanol solution with CTAB as surfactant, followed by the
addition of Au-PVP,17 then another portion of TEOS was added to the
above mixture followed by the addition of BTME as illustrated in
Scheme 1 (The detail of the preparation procedure could be found in
supplementary information). The hydrothermal treatment of the
mixture at 80 oC gives final yolk-shell nano-spheres,
SiO2@Au@Organosilica (Au@O-SiO2 Y-S). The TEM image of
Au@O-SiO2 Y-S clearly shows the yolk-shell nano-spheres with outer
particle size of ~500 nm and inner core size of ~200 nm (Figure 1a).
The pores on the shell were around 2 nm and Au NPs could clearly be
observed in the area much larger than the core, indicating that the
particles captured by SiO2 were released during the formation of the
shell. The uniform distribution of nano-spheres with particle size of
500 nm could be clearly observed in the SEM image of this sample
(Figure 1b). The distribution of the particle size in Figure 1 shows the
average diameter of Au-PVP confined in the yolk-shell nanospheres is
4.2 nm, which is similar to that of free Au-PVP (particle size in the
range of 3 nm ~ 5 nm, Figure S1). The X-ray diffraction pattern of
Au@O-SiO2 Y-S only affords one wide diffraction peak at 2-theta of
Figure 1. (a). TEM and (b). SEM image of Au@O-SiO2 yolk-shell. TEM images of
22 degree characteristic of the amorphous silica and no diffraction
peaks from Au NPs could be observed (Figure S2). The above results
indicate that Au NPs are distributed uniformly in the yolk-shell
nanospheres and no aggregation of Au NPs occurs during the
synthesis process. Based on nitrogen adsorption/desorption
measurement (Figure S3), Au@O-SiO2 Y-S has pore diameter of 2 nm,
BET surface area of 689 m2/g and pore volume of 0.42 cm3/g.
(c). SiO2 core formed at the first stage, (d). Au@SiO2 core-shell after adding Au-
PVP and second portion of TEOS, (e). SiO2@SiO2 core-shell formed without
adding Au-PVP, and (f). O-SiO2 hollow sphere formed without adding Au-PVP.
Styrene oxide is an industrially important organic intermediate used in
the synthesis of fine chemicals and pharmaceuticals. The traditional
preparation method involves stoichiometric oxidation using organic
peracides as oxidants, which is not only expensive but also inefficient
and environmentally unfriendly. The direct oxidation of styrene for
the formation of styrene oxide using O2 as oxidant is highly desirable.
Vladimir showed that gold clusters with 55 atoms could catalyze the
oxidation of styrene using oxygen gas with styrene oxide as the main
product, but the reactivity was moderate (conversions<30% on
different supports).20 Yu prepared Gold NPs stabilized by task-specific
ionic complexes.21 The optimized catalytic systems achieved 32.9%
styrene conversion with 72.0% selectivity to the epoxide and could be
reused four times without significant loss of activity. The catalytic
performance of Au@O-SiO2 Y-S was tested for styrene epoxidation
using oxygen as oxidant and was compared with Au-PVP, Au/O-SiO2
prepared by impregnation method and Au-PVP/O-SiO2 prepared by
directly loading of Au-PVP on O-SiO2 hollow spheres (Table 1). Since
Au-PVP NPs could not be distributed in dioxane, the control reaction
using Au-PVP as catalyst was carried on in water. Only 9.1% conv. was
observed on Au-PVP and the color of reaction solution turned from
light brown into clear after the reaction, indicating the aggregation of
Au NPs.17 Au@O-SiO2 Y-S could smoothly catalyze the reaction to
afford 49.4% conv. with selectivity to styrene epoxide of 66.1%,
For clarifying the formation mechanism, the TEM images of samples
taken at different synthetic stages are summarized in Figure 1c-f. In
the first stage, porous silica nanopsheres with a diameter of about 200
nm were formed. After the addition of Au-PVP, Au-PVP adheres on
the surface of silica nanopsheres for decreasing the surface energy.
The growth of a thin layer of SiO2 on Au/SiO2 nanospheres could be
clearly observed with the addition of second portion of TEOS for the
formation of Au@SiO2 core-shell nanospheres (Au@SiO2 C-S) with
particle size of around 300 nm (Figure 1d). After the BTME addition
and hydrothermal treatment, Au@SiO2 C-S nanospheres transforms
to Au@O-SiO2 Y-S nanospheres, which is mainly due to the etching
effect of organosilane precursor as we reported previously.18 The inner
core size of Au@O-SiO2 Y-S remains almost the same as the particle
size of SiO2 formed in the initial stage, showing silica layer formed
with the addition of second portion of TEOS was removed by the
addition of BTME. A control experiment shows that only organosilica
hollow nanospheres (O-SiO2 H-S) could be formed when no Au-PVP
was added with other synthesis conditions similar to Au@O-SiO2 Y-S
(Figure 1f). This is probably due to the protection effect of PVP which
could prevent the etching of silica core by BTME.19 This further
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Table.1 The catalytic performance of Au-PVP and solid catalysts for styrene oxidation using O2 as oxidanta
Sel. (%)
Particle size
(nm)
Au loading
(wt %)
Conv.
(%)
catalyst
1
2
3
Au-PVPb
Au@O-SiO2 Y-S
Au/O-SiO2
3 nm
4 nm
8 nm
3 nm
-
9.1 ± 0.7
49.4 ± 3.2
50.5 ± 5.3
10.4 ± 0.7
87.6 ± 4.0
27.2 ± 2.5
32.7 ± 3.2
64.5 ± 3.5
6.0 ± 1.0
6.4 ± 0.4
0.53 ± 0.02
0.42 ± 0.01
0.54 ± 0.01
6.6 ± 0.6 66.1 ± 2.6
6.8 ± 0.4 60.5 ± 2.9
3.3 ± 1.3 32.1 ± 3.2
d
Au-PVP/O-SiO2
a Reaction conditions: 5.5 mmol of styrene, 0.00127 mmol of Au for each catalyst, 3 ml of dioxane, 100 oC, 15 h, 1 atm O2; b 3 ml water was used as
solvent instead of dioxane; c prepared by impregnation method with O-SiO2 hollow spheres as support and HAuCl4 as metal source with reduction by
hydrogen at 120 oC; d prepared by directly loading of Au-PVP on O-SiO2 hollow spheres. 1.benzaldehyde; 2.acetophenone; 3.styrene oxide
showing that Au-PVP confined in yolk-shell nanospheres is efficient to would form when using solvent which could firstly react with active-
catalyze the oxidation of styrene. Au NPs loaded on silica hollow oxygen.6 The intermediate could convert into peroxide and serve as
nanospheres by traditional impregnation method resulted in Au/O- oxidant for the epoxidation reaction. From the time course, we can
SiO2 with Au particle size of 8 nm (TEM see Figure S4a-b). Under see that the benzaldehyde was the primary product at the beginning
similar reaction conditions the catalyst showed a similar initial activity of the reaction but the selectivity to styrene oxide increased with the
and selectivity towards styrene oxidation with Au@O-SiO2 Y-S with a conversion of styrene and finally became the main product. Thus, the
conversion of 50.5% and selectivity of 60.5% towards styrene oxide. induction time may be related with the conversion of a free radical
For Au-PVP /O-SiO2 as mentioned before the size of PVP protected intermediate into peroxide. The low selectivity to styrene epoxide on
gold nano-particles is larger than the pore size in the shell of hollow Au-PVP and Au-PVP/SiO2 may be due to either the solvent effect
nanospheres, so it is not surprising that O-SiO2 H-S could not provide (when water was applied to ensure the distribution of the Au-PVP) or
substantial shelter for the pre-synthesized catalysts. The activity was the aggregation of Au NPs during the catalytic process which led the
as low as its semi-homogeneous counterpart and the main product reaction stop at its early stage.
was benzaldehyde (10.4% conv. with 32.1% sel. to styrene oxide).
To further clarify the reusability of Au@O-SiO2 Y-S and Au/O-SiO2
To reveal the production of styrene oxide on Au@O-SiO2 Y-S, the both catalysts were tested by performing the reaction with the same
reaction process was monitored via sampling on time stream (Figure condition for more cycles. After each cycle the catalyst was separated
2a). From the time course plots, we can see that the reaction was via centrifugation, washed several times with ethanol and then dried.
carried on in a rather slow manner in the first 1 h. A conversion of No obvious deterioration of activity and selectivity was observed for
50.9% was achieved after 12 hours and acetophenone, benzaldehyde Au@O-SiO2 Y-S after even 8 cycles (Figure 2b). The conversion
and styrene oxide were the main products. The selectivity changed a showed a little bit increase after first four to five cycles. On TEM
lot during the process and styrene oxide was the main product in the images of the recovered catalyst (after 8th run, Figure S5) we can find
end, with a selectivity of 62.2%. It is worth noting that the inducing the fragments of broken nano-reactors, which may explain the
period could also be observed in the oxidation of cyclohexene improvements of activity for the fragments may facilitate the mass
catalyzed by Au NPs supported by mesoporous silica due to the transfer of both substrates and products. The size distribution of Au
growth in particle size during the catalytic process for the formation of NPs changed only slightly (averagely from ~4.2 nm to ~4.5 nm, Figure
active species.22 In our case no obvious growth in particle size was S5). High resolution Scanning Electronic Microscope was performed
observed as evidenced by the TEM image of the reused catalyst to further confirmed the distribution of Au NPs on the reused catalyst
(Figure S5). The previous studies show that a free radical intermediate (Figure S6). Nearly no Au NPs could be observed under scanning
Figure 2. (a) Time course plots of styrene oxidation on Au@O-SiO2 Y-S catalyst and (b) Reusability of Au@O-SiO2 Y-S (Filled) and Au/O-SiO2 (patterned) (the
upper , middle and bottom of the column are the yield for 1 styrene oxide, 2 acetophenone and 3 benzaldehyde, respectively) in the oxidation of styrene.
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mode but small bright spots could be seen in the same area when Electronic Supplementary Information (ESI) available: [details of any
scanning-transmitting mode was applied. The same phenomenon was supplementary information available should be included here]. See
observed even when we point the camera to the fragment which DOI: 10.1039/c000000x/
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