Postsynthetic functionalization of a hollow silica nanoreactor with manganese oxide-immobilized metal nanocrystals inside the cavity

Article abstract of DOI:10.1021/ja4083792

A postsynthetic protocol of functionalizing the preformed hollow nanoparticles with metal nanocrystals was developed based on galvanic replacement reaction on the Mn₃O₄ surface inside the cavity. The developed protocol produced hollow nanoreactor systems, in which a high density of ultrafine catalytic nanocrystals of a range of noble metals, such as Pd, Pt, Rh, and Ir and their alloys, are dispersively immobilized on an interior surface enclosed by a selectively permeable silica shell. The fabricated hollow nanoreactor exhibited highly enhanced activity, selectivity, and recyclability in catalyzing the oxidation of hydrosilanes, which are attributable to the synergistic combination of the porous silica nanoshell and the oxide-immobilized catalyst system.

Full text of DOI:10.1021/ja4083792

Communication
pubs.acs.org/JACS
Postsynthetic Functionalization of a Hollow Silica Nanoreactor with
Manganese Oxide-Immobilized Metal Nanocrystals Inside the Cavity
Soo Min Kim,† Mina Jeon,† Ki Woong Kim,‡ Jaiwook Park,† and In Su Lee*,†
†Department of Chemistry, Pohang University of Science and Technology (POSTECH), Gyeogbuk 790-784, Korea
‡Department of Applied Chemistry, Kyung Hee University, Gyeonggi-do 446-701, Korea
S
* Supporting Information
generated void space.9 In this context, we report our research
ABSTRACT: A postsynthetic protocol of functionalizing
the preformed hollow nanoparticles with metal nanocryst-
als was developed based on galvanic replacement reaction
on the Mn3O4 surface inside the cavity. The developed
protocol produced hollow nanoreactor systems, in which a
high density of ultrafine catalytic nanocrystals of a range of
noble metals, such as Pd, Pt, Rh, and Ir and their alloys, are
dispersively immobilized on an interior surface enclosed by
a selectively permeable silica shell. The fabricated hollow
nanoreactor exhibited highly enhanced activity, selectivity,
and recyclability in catalyzing the oxidation of hydro-
silanes, which are attributable to the synergistic combina-
tion of the porous silica nanoshell and the oxide-
immobilized catalyst system.
intended to exploit a postsynthetic modification method to
functionalize preformed void spaces of the hollow nano-
particles, which might be more desirable for enhancing the
variability in the number and the nature of entrapped catalytic
nanoparticles. For this purpose, we attempted to further
develop our recent discovery that Pd and Pt nanocrystals are
spontaneously deposited on the Mn3O4 surface from their
precursor solutions, through the galvanic replacement process
without the use of any additional reducing agents.10 It was
envisioned that if this process is properly taking place in the
interior cavity, it could be a simple and efficient method for
decorating the interior surface of the preformed hollow
nanostructure with large multiple numbers of catalytic
nanoparticles, which are more advantageous for catalytic
applications than is a yolk-shell nanostructure with a single
large core.11
Herein, we report a postsynthetic functionalization protocol
based on galvanic replacement at the Mn3O4 surface, which
leads to the high-density deposition of ultrafine metal
nanoparticles on a Mn3O4-layer-coated interior surface of the
hollow silica nanosphere (Scheme 1). The hollow nanoreactor
ollow inorganic nanoparticles, which have an interior
Hcavity enclosed by an inorganic shell of nanosized
thickness, have recently been receiving considerable attention
because of their distinct characteristics that are advantageous
for a variety of biomedical and catalytic applications.1 Their
hollow interior structure can be used to selectively encapsulate
and release guest molecules and carry the high payload of the
functional molecules on the large surface area, which therefore
enriches their potential in the emerging fields such as
nanoreactors,2 drug delivery vehicles,3 contrast agents for
molecular imaging,4 and energy and gas storage materials.5 In
particular, hollow nanostructures that encapsulate catalytic
species inside the permeable porous shell of chemically inert
materials are promising candidates for nanoreactors that
efficiently catalyze the chemical reactions of selected molecules
while preserving the exposed surface area of the entrapped
catalysts even under harsh reaction conditions or during the
recycling process.6,7 To increase the potential applications of
such nanoparticles in nanoreactors, it is essential to develop a
new method for functionalizing the internal cavity, which allows
chemical reactions to occur within the protective void space.
Several effective methods have been proposed to incorporate
catalytic nanoparticles inside the cavity of the hollow
nanoparticles through various approaches, including the
template etching process and methods based on the Ostwald
ripening, galvanic replacement, and nanoscale Kirkendall
effects.8 Most of the above-mentioned methods are used to
produce yolk-shell nanostructures by selectively removing a
part of a priorly synthesized core−shell nanostructure, which
leaves a single metal nanoparticle core inside the newly
Scheme 1. Protocol of Functionalizing the Interior Surface
of a Hollow Silica Nanoparticle
fabricated using the proposed method exhibited highly
enhanced activity, selectivity, and recyclability in catalyzing
the hydrolytic oxidation of silanes, which are attributable to the
synergistic combination of the porous silica nanoshell and the
oxide-immobilized catalyst system. We also demonstrated the
broad scope of the proposed protocol to diversify the functions
of the hollow nanoreactor by introducing various catalytic metal
species, including Pd, Pt, Rh, Ir, and their alloys.
The hollow silica nanosphere with a Mn3O4 layer on the
interior surface, HMON@h-SiO2, which was used as a platform
in this study, was prepared using a recently developed
procedure involving silica encapsulation of a MnO−Mn3O4
Received: August 13, 2013
Published: October 8, 2013
© 2013 American Chemical Society
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Communication
core−shell nanoparticle (CSNP), followed by treatment with
hydroxylamine solution for etching the MnO core and
rendering the silica porous.12 The resultant HMON@h-SiO2
obtained from 16 ( 1) nm-sized CSNP consisted of a porous
and hollow silica nanoshell with an 8 ( 1) nm diameter cavity
and a 4.3 ( 0.5) nm Mn3O4 layer on the interior surface
(Figure 1a). To induce Pd deposition on the Mn3O4 layer,
nanoparticles, which accompanies the partial dissolution of
the Mn3O4 layer, was identified by TEM observations of the
resulted particles. The control experiment with hollow silica
nanoparticles prepared from HMON@h-SiO2 by dissolving
Mn3O4 did not give any Pd species, which confirmed that the
2−
reduction of PdCl4 proceeded through the galvanic reaction
with Mn3O4 inside the permeable silica shell (Figure S2).
The results of TEM analyses also revealed that a low pH,
which may prevent the deposition and precipitation of Pd2+
species on the silica surface, is essential for the growth of Pd
nanocrystals exclusively on the interior surface.14 Thus, the
reaction of HMON@h-SiO2 having a shell thickness of 8.5 nm
2−
with PdCl4 at pH 2.4 led to Pd deposition on the Mn3O4
layer on the interior surface, while the treatment at pH > 2.5 led
to Pd growth outside the silica shell (Figure S3). TEM images
recorded at different instances during the reaction at pH 2.4
showed that tiny Pd nanocrystals with an average size of 1.6
( 0.4) nm began to emerge on the Mn3O4 layer at 5 min
(Figure 1b). As the reaction proceeded, slightly larger Pd
nanocrystals were additionally deposited, which is consistent
with the previous observation during the galvanic replacement
reaction on Sf-MON (Figure 1c,d). Therefore, the SEM, TEM,
and STEM elemental mapping images obtained after a reaction
time of 30 min revealed the formation of the targeted HMON/
Pds@h-SiO2 (Figure 1e−g). In this product, 2.0 ( 0.4) nm-
sized Pd nanocrystals were densely and dispersively deposited
on the partially dissolved Mn3O4 layer, on the internal surface
of a porous and hollow silica nanoshell, whose inner and outer
diameters were 16 ( 2) and 35 ( 2) nm, respectively. The
nitrogen adsorption/desorption isotherm analysis shows that
the resulted HMON/Pds@h-SiO2 has 287 m2/g of a BET
surface area and micropores in the silica shell with a bimodal
size distribution each centered at 0.7 and 1.4 nm (Figure S4).
The Pd and Mn contents of HMON/Pds@h-SiO2 were
determined to be 2.5 and 5.1 wt %, respectively, by using
inductively coupled plasma atomic emission spectroscopy
(ICP-AES). Thus, HMON/Pds@h-SiO2, in which an oxide-
immobilized nanocrystalline-catalyst system is confined to the
void space protected by a selectively permeable silica shell, can
efficiently catalyze the transformation of the selected molecules
and can be recycled many times without loss of activity.
In order to verify the effectiveness of HMON/Pds@h-SiO2
as a nanoreactor, we investigated its catalytic performance in
the hydrolytic oxidation of hydrosilanes (1), which is known to
be facilitated by noble metal-based catalysts (Table 1).15 In the
trial reaction of dimethylphenylsilane (1b) and water (3 equiv)
with 1 mol % of HMON/Pds@h-SiO2 in acetone at room
temperature (rt), dimethylphenylsilanol (2b) was afforded as
the sole product in quantitative yield within 30 min (entry 3).
In contrast, the control reaction with the same wt% of
HMON@h-SiO2 afforded only 2% conversion yield for 24 h,
implying that the Pd nanocrystals inside the silica shell are
responsible for the catalytic activity of the HMON/Pds@h-
SiO2. The turnover number (TON) of the HMON/Pds@h-
SiO2 estimated based on the conversion yield with 0.0005 mol
% of the catalyst for over 6 h was >198 000, which is highest
among those reported so far for silane oxidation reactions with
heterogeneous catalysts.15a The control reactions by using
commercialized Pd/CaCO3, Pd/Al2O3, and Pd/C catalysts,
which immobilize 6 ( 2), 3.7 ( 0.6), and 3.2 ( 0.6) nm-sized
Pd nanoparticles, respectively, gave only 10 000−36 000 of
TONs, which confirms the highly enhanced activity of the
HMON/Pds@h-SiO2 (Figure S5). Although the exact
Figure 1. TEM and HRTEM (inset) images of (a) HMON@h-SiO2
having a shell thickness of 8.5 nm and samples obtained after its
reaction with Na2PdCl4 at pH 2.4 for (b) 5, (c) 10, (d) 20, and (e) 30
min. Histograms are showing the outer diameter and the thickness of
the silica shell and the size of the deposited Pd nanocrystals. (f) SEM
and (g) STEM-HAADF image and elementary maps of HMON/
Pds@h-SiO2 obtained after 30 min reaction.
HMON@h-SiO2 was immersed in an aqueous solution of
Na2PdCl4 at 70 °C by following a procedure that was used for
the surfactant free Mn3O4 nanoparticle (Sf-MON).10 The
results of initial experiments revealed that the thickness of the
silica shell, which varies between 7.5 and 12.5 nm with the
duration of the silica encapsulation procedure, is a critical factor
in the reaction of PdCl42− and Mn3O4. Therefore, HMON@h-
SiO2 with a shell thickness >11.7 ( 0.9) nm, which was
prepared through silica encapsulation for longer than 12 h, did
not generate any reduced Pd species from the solution,
presumably because the reducing Mn3O4 layer was shielded by
a relatively thicker and denser silica shell (Figure S1).13 In
contrast, immersion of HMON@h-SiO2s with shell thicknesses
of 7.5, 8.5, and 10.3 nm caused gradual darkening of the
2−
suspension color, indicating the reduction of PdCl4 into
metallic Pd species, and resulted in translucent dark-brown
suspensions within 30 min. The formation of tiny Pd
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To examine the possibility of expanding the current
procedure to a broader range of catalytic species besides Pd
and Pt, further investigation was performed on the metal
growth with Sf-MON. Immersion of Sf-MON in IrCl3 and
RhCl3 solutions resulted in the formation of Ir and Rh
nanocrystals, respectively, on the Sf-MON surface, similar to Pd
and Pt deposition through the galvanic replacement process. In
addition to the monometallic nanocrystals, their alloys such as
binary Pd/Pt and quaternary Pd/Pt/Ir/Rh could be deposited
by treating Sf-MON with solutions containing the correspond-
ing precursor mixtures. The resultant nanocomposites, which
immobilize the monometallic and alloy nanocrystals on the Sf-
MON surface, were identified by using TEM, HR-TEM, and
STEM elemental mapping (Figures S8 and S9). Following the
successful validation of the extendibility of the galvanic
replacement process, we investigated the possible decoration
of the hollow nanoreactor with various catalytic species. The
reactions of HMON@h-SiO2 with Na2PtCl4, IrCl3, and RhCl3
solutions resulted in the growth of Pt, Ir, and Rh nanocrystals
with sizes of 1.9 ( 0.4), 1.5 ( 0.2), and 1.5 ( 0.3) nm,
respectively, on the Mn3O4 layer inside the cavity (Figure 2).
Table 1. Oxidation of Hydrosilanes into Silanols with
HMON/Pds@h-SiO2 as a Catalyst
a
b
bc
,
entry
silane (1)
time [h] conv. [%] yield [%]
TOF [h−1
]
1
2
3
4
5
Et3SiH (1a)
PhMe2SiH (1b)
1b
0.25
0.25
0.5
95
70
93 (2)
380
280
69 (<1)
98 (1)
95 (5)
60 (2)
>99
>99
62
d
1b (10th run)
0.5
Ph2MeSiH (1c)
0.25
3
248
17
e
f
g
6
Ph3SiH (1d)
>99
98(2)
a
Condition: 1 (0.5 mmol), H2O (1.5 mmol), acetone (1.5 mL),
HMON/Pds@h-SiO2 (1 mol %, based on Pd contents), rt, under air.
b
c
Determined by GC. Nnumbers in parentheses represent the yield of
d
e
the product 3. Recovered catalyst was used in iterative cycles. 2 mol
% catalyst was used. Conversion was not observed during the reaction
with 1 mol % catalyst for 0.25 h. Isolation yield.
f
g
explanation requires further research, such a remarkable
catalytic activity can be ascribed to both the ultrafine size and
the large number of catalytic nanoparticles that are well
dispersed on the Mn3O4 support layer and the possible
synergistic support-catalyst interaction at the Mn3O4-Pd
heterojunction. Reuse of the recovered HMON/Pds@h-SiO2
in 10 consecutive runs did not lead to any significant decline in
its catalytic activity in terms of conversion yield and selectivity,
indicating the high recyclability of the hollow nanoreactor
carrying the immobilized catalyst system inside the cavity
(entry 4). The high dispersion stability of the Mn3O4-
immobilized Pd nanocrystals, presumably resulting from the
strong metal-support interaction, was also confirmed by
treating solid HMON/Pds@h-SiO2 at high temperatures; the
initial highly dispersed state was retained even after annealing in
vacuo at 200 °C for 12 h (Figure S6). The catalytic turnover
frequency (TOF) of the HMON/Pds@h-SiO2 for the
oxidation of 1b was estimated to be 280 h−1 based on 70%
of conversion yield during the reaction for 15 min (entry 2).
Compared with this, the same reaction with triethylsilane (1a)
and diphenylmethylsilane (1c) gave TOFs of 380 and 248 h−1,
respectively (entries 1 and 5). In case of the triphenylsilane
(1d), the TOF was derived to be only 17 h−1 from the reaction
performed with 2 mol % catalyst for 3 h (entry 6). The
measured TOF values show good correlation with the
molecular size of the substrate hydrosilane molecules,
indicating that the catalytic conversion inside the hollow cavity
of HMON/Pds@h-SiO2 is largely influenced by the diffusion
rate of the substrate molecules through the pores of the silica
shell.16 Control reactions using Mn3O4@Pds, in which Pd
nanocrystals are immobilized on the Sf-MON surface, showed
the quantitative transformations of 1a∼c and 1d within 12 and
40 min, respectively. This result evidences the crucial role of
the porous silica shell in enhancing the substrate selectivity of
the hollow nanoreactor by sorting the substrates according to
molecular size. On the other hand, in the experiments to
investigate the TON by using 0.0005 mol % of the catalyst, the
Pd nanocrystals immobilized in the Mn3O4@Pds were found to
be aggregated and sintered into larger particles, while any
change was detected from the Pd nanocrystals immobilized
inside the cavity of the retrieved HMON/Pds@h-SiO2 after the
catalytic reaction (Figure S7).
Figure 2. TEM and HRTEM (inset) images of (a) HMON/Pts@h-
SiO2, (b) HMON/Irs@h-SiO2, and (c) HMON/Rhs@h-SiO2.
Histograms show the outer diameter and the size of the deposited
metal nanocrystals.
The resulting HMON/Ms@h-SiO2 (M = Pt, Ir, and Rh) had a
functionalized interior surface with a high density of catalytic
nanocrystals. Furthermore, hollow nanoreactors, which incor-
porate alloy nanocrystals such as Pd/Pt and Pd/Pt/Rh/Ir in the
hollow interior surface, were successfully fabricated by treating
HMON@h-SiO2 in the solutions of the precursor complexes
(Figure 3).
In summary, a postsynthetic method for functionalizing
preformed hollow nanoparticles with catalytically active metal
nanocrystals was developed based on galvanic replacement on
the Mn3O4 surface inside the cavity. Our method produced
hollow nanoreactor systems, in which a high density of ultrafine
catalyst nanocrystals are dispersively immobilized on the
Mn3O4-coated interior surface enclosed by a selectively
permeable silica shell. The high effectiveness of the fabricated
nanoreactor was demonstrated in terms of its activity,
selectivity, and recyclability in catalyzing the silane oxidation
reaction. Furthermore, we demonstrated that the procedure
explored in this study could be extended into a generalized
protocol that is applicable for optimizing and diversifying the
functionality of the hollow nanoreactor by combinatorial
integration of a wide range of catalytic species inside the cavity.
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Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
* Supporting Information
Experimental details and additional analyses data. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
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(12) Anisur, R. M.; Shin, J.; Choi, H. H.; Yeo, K. M.; Kang, E. J.; Lee,
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AUTHOR INFORMATION
Corresponding Author
insulee97@postech.ac.kr
Notes
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■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This work was supported by the National Research foundation
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(MEST) (2011-0017377). We thank Dr. N.-S. Lee at
POSTECH NCNT for TEM analyses.
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