Silica nanosphere-supported shaped Pd nanoparticles encapsulated with nanoporous silica shell: Efficient and recyclable nanocatalysts

Article abstract of DOI:10.1039/c0jm01093f

A synthetic method to a highly efficient heterogeneous nanocatalyst, consisting of silica nanosphere (SiO₂) decorated with palladium nanoparticles (Pd-NP) that are further encapsulated within a nanoporous silica shell (Porous-SiO₂),

Full text of DOI:10.1039/c0jm01093f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Silica nanosphere-supported shaped Pd nanoparticles encapsulated with
nanoporous silica shell: Efficient and recyclable nanocatalysts†
c
ab
Yanfei Wang,ab Ankush V. Biradar,ab Cole T. Duncan and Tewodros Asefa
*
Received 19th April 2010, Accepted 24th June 2010
DOI: 10.1039/c0jm01093f
A synthetic method to a highly efficient heterogeneous nanocatalyst, consisting of silica nanosphere
(SiO2) decorated with palladium nanoparticles (Pd–NP) that are further encapsulated within
a nanoporous silica shell (Porous-SiO2), is reported. First, monodisperse, 5 nm and 20 nm Pd
nanoparticles are synthesized and anchored onto silica nanospheres of ꢀ250 nm in diameter. These
core-shell nanospheres are then coated with a secondary silica shell by the sol–gel process. This is
followed by controlled etching of the outer silica shell to yield nanoporous silica shell around the Pd–
NP. The thickness of the nanoporous silica shell and its pore structures are controlled by changing the
synthetic conditions. The resulting nanoporous silica shell is proved to permit reactants to reach the
Pd–NP while at the same time protect them from aggregation. These new nanomaterials, dubbed SiO2/
Pd Nanoparticles/Nanoporous SiO2 (or SiO2/Pd–NP/Porous-SiO2) core-shell-shell nanospheres
behave as heterogeneous nanocatalyst exhibiting high catalytic activity and turn-over-numbers (TONs)
in hydrogenation reaction of various substrates at room temperature and 20 bar hydrogen pressure.
The core-shell-shell nanospheres are proven to be versatile catalysts as they are also able to catalyze
C–C coupling reactions effectively. Moreover, these heterogeneous nanocatalysts are stable showing
negligible Pd leaching and aggregation, and can be recycled multiple times without loss of catalytic
activity.
many metal nanoparticles can not attain their maximum possible
Introduction
catalytic activity and selectivity either because of the presence of
their surface passivating ligands or because of their aggregation
without them.13–15 In particular, Pd–NP catalysts are known to
aggregate easily into bulk Pd precipitate without strong capping
agents around them.16–20 Therefore, the development of highly
efficient, stable Pd-based heterogeneous catalysts containing
‘‘naked nanoparticles’’ or nanoparticles with loosely bound
surface ligands still remains challenging.
Palladium-based heterogeneous catalysts have long received
great attention owing to their ability to catalyze a variety of
useful chemical reactions as well as their advantage of easy
recovery and regeneration.1–6 Because of their high surface areas
and greater stability, nanostructured materials have recently
attracted a great deal of interest for supporting active catalytic
sites including metallic nanoparticles such as Pd nanoparticles
(Pd–NP) for the preparation of efficient heterogeneous cata-
lysts.7–11 Pd–NP have now become among the most interesting
catalysts because of their size and shape-dependent as well as
efficient catalytic activities in a number of reactions.12 The most
efficient catalytic activity for Pd–NP, and also other metallic
nanomaterials, could be ideally attained if the nanomaterials’
surface reactive sites were ‘‘naked’’. However, without surface
capping agents and stabilizers, the surface atoms of metallic
nanoparticles with their typically high surface energy become
susceptible to aggregation into bulk material. Consequently,
To prepare heterogeneous catalysts with metallic nano-
particles including Pd–NP or to prevent nanoparticles from
coalescence, various types of inorganic shells such as silica,21–23
carbon9,24 and metal oxides25,26 around the nanoparticles can be
used. Such metal/inorganic type core–shell nanostructures are
often generated by using two sequential synthetic steps; i.e. the
preparation of metal nanoparticles followed by the deposition of
the inorganic material around them using appropriate synthetic
methods.27,28 For instance, the synthesis of metal-silica core-shell
nanospheres with a single metal nanoparticle core that is coated
€
isotropically by a dense silica shell via the well-known Stober
method has been reported.29 Very recently, the synthesis of core-
shell nanosphere catalysts consisting of a single Pd nanoparticle
core encapsulated within a porous silica shell has also been
achieved.1 However, these core-shell nanospheres have poly-
disperse size distributions and most of them contain only one
metal nanoparticle per each core-shell nanosphere.1,22,29 In light
of the reports by El-Sayed and others that a large number of edge
and corner atoms in metal nanocrystals is required to enhance
their catalytic performance,30,31 we conceived that utilizing
smaller and shaped nanoparticles in large number per a single
core-shell-shell type nanosphere having a nanoporous shell may
aDepartment of Chemistry and Chemical Biology, Rutgers, The State
University of New Jersey, 610 Taylor Road, Piscataway, New Jersey,
08854, USA. E-mail: tasefa@rci.rutgers.edu
bDepartment of Chemical and Biochemical Engineering, Rutgers, The State
University of New Jersey, 98 Brett Road, Piscataway, New Jersey, 08854,
USA. E-mail: tasefa@rci.rutgers.edu
cDepartment of Chemistry, Syracuse University, 111 College Place,
Syracuse, New York, 13244, USA
† Electronic supplementary information (ESI) available: Transmission
microscopy (TEM) images, gas adsorption isotherm, pore size
1
distribution, X-ray diffraction (XRD) patterns, H NMR spectra, and
graph showing thickness of silica shell versus alkoxysilane
concentration. See DOI: 10.1039/c0jm01093f
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result in better catalytic performance. We have investigated this
concept using Pd–NP.
The resulting –NH2 terminated silica nanospheres were then
dispersed in de-ionized water (20 mL).
Herein, we report the successful synthesis of such novel core-
shell-shell nanospheres, with each nanosphere containing several
octahedral-shaped 5 nm or 20 nm Pd–NP sandwiched between
a silica nanosphere core and a nanoporous silica shell (Porous-
SiO2). The resulting materials are labeled as SiO2/Pd–NP/
Porous-SiO2 core-shell-shell nanospheres. By using these SiO2/
Pd–NP/Porous-SiO2 core-shell-shell nanospheres as hetero gen-
eous catalyst, we have demonstrated highly efficient and recy-
clable catalytic hydrogenation and C–C coupling reactions at
room temperature.32 Furthermore, the Pd–NP sandwiched
between the silica core and the nanoporous silica shell showed
good stability with no Pd leaching and particle aggregation.
Synthesis of SiO2/Pd–NP core-shell nanospheres
Pd–NP were adsorbed onto the –NH2 functionalized silica
nanospheres by adding the above –NH2 terminated SiO2
nanospheres (4.8 mL) into the solution containing Pd–NP (12
mL, 7.3 mM on the basis of Pd atoms) that was further
diluted in 125 mL de-ionized water under sonication. The
solution was allowed to equilibrate for 10 min. The colloidal
solution was then centrifuged and the precipitate was washed
with water once and dispersed in de-ionized water (8.6 mL).
This gave
a colloidal solution of SiO2/Pd–NP core-shell
nanospheres.
Experimental
Synthesis of SiO2/Pd–NP/SiO2 core-shell-shell nanospheres
Materials and reagents
The solution containing SiO2/Pd–NP nanospheres from above (2
mL) was dispersed in a mixture of ethanol (100 mL), water (12.5
mL), and ammonium hydroxide solution (5 mL). Then, 0.25 mL
TEOS was added into this solution to encapsulate the SiO2/Pd–
NP with a silica shell. After 12 h of stirring, the solution was
centrifuged and the precipitate was washed with ethanol. The
final precipitate was dispersed in de-ionized water (25 mL) giving
a colloidal solution of SiO2/Pd–NP/SiO2 core-shell-shell nano-
spheres.
Anhydrous ethanol, aqueous ammonium hydroxide solution
(28%), citric acid and sodium hydroxide (Certified, A.C.S.) were
purchased from Fisher Scientific. 3-Aminopropyltriethoxysilane
(APTS, 99%) was obtained from Acros Organics. Tetraethyl
orthosilicate (TEOS, 98%), sodium tetrachloropalladate(II)
(Na2PdCl4), polyvinylpyrrolidone (PVP, MW ¼ ꢀ55000), and
dimethyl sulfoxide (DMSO) were obtained from Sigma-Aldrich.
Polyvinylpyrrolidone K15 (PVP K15, Mw ꢀ10000) was
purchased from Fluka. Unless mentioned otherwise, Millipore
water with a high resistivity of 18 M-U was used for the synthesis
of the materials.
Etching of silica shell of SiO2/Pd–NP/SO2 nanospheres
To etch the silica shell into nanoporous silica (Porous-SiO2), 1 g
of PVP K15 was dissolved in the solution containing SiO2/Pd–
NP/SiO2 nanospheres (25 mL) under stirring. This solution was
then refluxed at 100 ꢁC for 3 h. After cooling down the solution
to room temperature, NaOH (0.8 g) was added into the solution
and stirred for 3 h under ambient condition to etch the silica
shell. After centrifugations and washing with distilled water and
ethanol three times in each case, SiO2/Pd–NP/Porous-SiO2 core-
shell-shell nanospheres were obtained. The particles were finally
dispersed in ethanol for TEM imaging and in methanol for
catalysis studies.
Synthesis of Pd nanoparticles (Pd NP)
Octahedral shaped Pd–NP were synthesized as reported previ-
ously.31 In a typical synthesis, PVP and citric acid (275 mg) were
dissolved in water (8.0 mL) in a 25 mL, three-necked flask con-
taining a Teflon-coated magnetic stir bar and refluxed at 90 ꢁC in
air with stirring. Sodium tetrachloropalladate(II) (7.3 mM) was
dissolved in water (3.0 mL) at room temperature, and this
aqueous solution was then rapidly added into the flask with
a pipette. The molar ratio of Na2PdCl4 to PVP was kept at 1 : 5
in all the experiments. The reaction mixture was heated at 90 ꢁC
in air for 3 h to obtain 5 nm Pd–NP or 26 h to obtain 20 nm
Pd–NP.
Catalytic hydrogenation reaction
The catalytic hydrogenation reactions were carried out in a 250
mL high-pressure reactor supplied by Parr Instrument
Company. The reactor was equipped with a heating arrange-
ment, overhead stirrer, thermowell, and pressure gauge as well as
transducer, gas inlet, gas outlet, and sampling valve. The reactor
has the provision to set temperatures and agitation speeds
Synthesis and surface functionalization of monodisperse silica
nanospheres (SiO2)
Monodisperse silica nanospheres were synthesized by following
€
29
the Stober method. Typically, 6.2 mL of TEOS was added to
6.5 mL of ammonium hydroxide solution (28 wt%) in 100 mL of
ethanol and 7.1 mL of Millipore water under stirring to hydro-
lyze TEOS. After 15 h of stirring, a colloidal solution of silica
nanospheres of 250 nm diameter was obtained. After washing
with ethanol through centrifugation and redispersion two times,
1 g of the silica nanospheres was transferred into a mixtureꢁof
isopropanol (180 mL) and APTS (1.5 mL) and stirred at 80 C
for 2 h to functionalize the silica surface with amino groups.33
After washing with isopropanol twice via centrifugation and
decantation, –NH2 terminated silica nanospheres were obtained.
through controllers. In a typical hydrogenation reaction,
a known amount of substrate was dissolved in solvent and the
etched SiO2/Pd–NP/Porous-SiO2 catalyst was charged into the
reactor. It should be mentioned that the size of Pd–NP in the
SiO2/Pd-NP/Porous-SiO2 used for all catalytic experiments was 5
nm. Typically, 8.73 mmol substrate and 10 mg etched SiO2/Pd–
NP/Porous-SiO2 nanospheres (3.59 wt.% Pd) in 40 mL methanol
at 20 bar H2 pressure were used. The reactor was flushed thrice
with nitrogen and then thrice with hydrogen before every reac-
tion. Hydrogen gas was introduced into the reactor to the desired
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pressure after setting the stirrer to slow stirring rate for uniform
heat and reactant distribution. After attaining the desired
temperature, the reaction was initiated by increasing the agita-
tion speed to 550 rpm. The consumption of hydrogen gas due to
the reaction was monitored from the pressure drop in the reactor.
The reaction was stopped when the gas consumption ceased. The
reactor contents were cooled, and the liquid sample was analyzed
1
with gas chromatography (GC), GC–MS, and H NMR spec-
troscopy.
Heck coupling reaction
In a 25 mL round bottom flask, SiO2/Pd–NP/Porous-SiO2
catalyst (10 mg) was dispersed in DMSO (10 mL) via sonication.
Styrene (1.3 mmol), iodobenzene (1.0 mol), and Et3N (1.5 mmol)
were then added and the reaction was stirred at 120 ꢁC. The
formation and yield of trans-stilbene were monitored by GC–
Scheme 1 Schematic representation of the synthesis of SiO2/Pd–NP/
Porous-SiO2 core-shell-shell nanospheres.
MS, GC and H NMR.32
1
particularly the etching time, the pore structure of the silica shell
was tuned or optimized.
Instrumentations
Representative TEM images of some of the materials
described in Scheme 1 are shown in Fig. 1 and in electronic
supporting information (ESI†), Figures S1 and S2. The more
magnified TEM images in Fig. 1 and the less magnified TEM
images in Fig. S1,II† show that the 20 nm Pd–NP are randomly
dispersed around the SiO2 nanospheres. Then, the coating of
silica shell with controlled thickness around the Pd–NP is ach-
ieved reproducibly by stirring the SiO2/Pd–NP nanospheres in
different concentrations of TEOS as shown in Fig. 1b and
S2,II,b. The thickness of the silica shell was found to depend
linearly on the amount of TEOS used for a given NH4OH
concentration and deposition time. For instance, TEM images
show an increase of thickness of the silica shell from 20 nm to 100
nm upon increasing the amount of TEOS from 0.1 mL to 0.6 mL
in the solution (Figure S3†). Interestingly, when the amount of
TEOS was increased to 0.6 mL, in addition to thicker silica shells,
some smaller Pd–NP/SiO2 type core-shell nanospheres also
formed (Figure S3,d†). The use of an optimum amount (0.25 mL)
of TEOS guaranteed the formation of a uniform 50-nm-thick
silica shell (Fig. 1b). In addition to the amount of TEOS, it is
worth mentioning that the thickness of the silica shell can also
depend on the reaction time and concentration of NH4OH.29
Higher concentration of TEOS or large amount of NH4OH can
lead to thicker silica shells for a given deposition time.29
Nitrogen gas adsorptions were measured with Micromeritics
Tristar 3000 volumetric adsorption analyzer after degassing the
samples at 160 ꢁC for 12 h. The TEM images were taken by using
an FEI Tecnai T-12 S/TEM instrument. The samples for TEM
analysis were prepared by sonicating the samples in ethanol for 3
min, casting a few drops of the solution on a formvar-carbon
coated copper grid and letting the solution dry under ambient
conditions. The GC was performed on Agilent HP 6850 con-
taining a flame ionization detector and an HP-1, MS 30 m ꢂ
0.200 mm ꢂ 0.25 mm capillary column. The GC-MS experiment
was conducted with HP-5890 equipped with 5972 MSD and an
HP-5 MS 50 m ꢂ 0.200 mm ꢂ 0.33 mm capillary column. The 1H
NMR spectra of the reaction mixtures were obtained with
€
a Bruker DPX-300 NMR spectrometer.
Results and discussion
The SiO2/Pd–NP/Porous-SiO2 core-shell-shell nanospheres,
consisting of shaped Pd nanoparticles (Pd–NP) sandwiched
between a silica nanosphere core and a nanoporous silica shell,
were prepared in four steps as illustrated in Scheme 1. In step 1,
silica nanospheres were synthesized and used as a support core
material for Pd–NP. To perform this, nearly monodisperse, 250-
€
nm silica nanospheres (SiO2) were synthesized using the Stober
The nanoporous silica shell around the Pd–NP was achieved
by selective etching of the outer silica shell with NaOH solution
in the presence of PVP.35 PVP was added in this step not to
stabilize the Pd–NP but rather to enable uniform in depth etching
or continuous exfoliation of the inner part of the silica nano-
spheres. Without PVP, only the outermost layer of the silica shell
will get etched predominantly resulting in non-uniformly etched
or a rather non-porous silica shell. The PVP helps the uniform
etching of the cores of silica or slows the etching of the outer
surface by forming strong interaction with the outermost surface
of the particles.35 Fig. 1c, Figure S2,II,c, and Figure S2† show
a more magnified, a less magnified and a large area TEM image,
respectively, of SiO2/Pd–NP/Porous-SiO2 nanospheres that were
etched for 120 min. As can be seen from these TEM images,
multiple Pd–NP are sandwiched between a solid silica nano-
particle core and a nanoporous silica shell. In addition to PVP,
method,29 and their surfaces were modified with 3-amino-
propyltrimethoxysilane (APTS).30 Then, octahedral shaped, ꢀ5
nm (or ꢀ 20 nm) Pd–NP were chemisorbed on the aminopropyl-
terminated silica nanospheres in Step 2, resulting in SiO2/Pd–NP
core-shell nanospheres. The octahedral Pd–NP were synthesized
by reducing different concentration of aqueous solution of Pd
salt with citric acid (Figure S1,I†).31 In Step 3, the SiO2/Pd–NP
core-shell nanospheres were coated with a silica shell producing
SiO2/Pd-NP/SiO2 core-shell-shell nanospheres. The deposition of
the silica shell around the SiO2/Pd–NP was performed by the sol–
gel process using tetraethyl orthosilicate (TEOS) as the silica
source in ethanol.34 Finally, in Step 4, controlled etching of the
outer silica shell with an aqueous NaOH solution generated
a nanoporous silica shell and SiO2/Pd–NP/Porous-SiO2 core-
shell-shell nanospheres. By changing the etching conditions,
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changing the etching time allows tuning the pore sizes and
structures of the porous silica shell. For instance, upon
increasing the etching time from 50 to 100 min, SiO2/Pd–NP/
Porous-SiO2 core-shell-shell nanospheres with concentric pores
around the Pd–NP formed (Fig. 2). Longer etching times of 150
min resulted in complete dissolution or removal of the second
silica shell (not shown). SiO2/Pd–NP/Porous-SiO2 nanospheres
with a uniform nanoporous shell were obtained by using optimal
etching time of 80 min. The resulting etched SiO2/Pd–NP/
Porous-SiO2 core-shell-shell nanospheres were readily dispersible
in water and remained stable for several months. In addition to
20 nm Pd–NP, the synthesis was proven to work well for 5 nm
Pd–NP producing SiO2/Pd–NP/Porous-SiO2 core-shell-shell
nanospheres (Figure S4†).
The nanoporosity of the silica shell of the SiO2/Pd–NP/Porous-
SiO2 nanospheres was confirmed by nitrogen gas adsorption.
Fig. 3 shows representative nitrogen gas adsorption/desorption
isotherms with the corresponding pore size distribution of the
sample displayed in the inset for SiO2/Pd–NP/Porous-SiO2
sample etched for 80 min. A Brunauer–Emmett–Teller (BET)
surface area and single-point total pore volume of 115 m2 gꢃ1 and
Fig. 1 TEM images of (a) SiO2/Pd–NP core-shell nanospheres with 20
nm Pd–NP chemisorbed on aminopropyl-modified silica nanospheres
and the corresponding (b) SiO2/Pd–NP/SiO2 core-shell-shell nano-
spheres, and (c) SiO2/Pd–NP/Porous-SiO2 core-shell-shell nanospheres
etched for 120 min.
Fig. 2 TEM images of SiO2/Pd–NP/SiO2 core-shell-shell nanospheres
(a) and the corresponding SiO2/Pd–NP/Porous-SiO2 core-shell-shell
nanospheres after etching for: (b) 50, (c) 60, (d) 70, (e) 80 and (f) 100 min.
Scale bars ¼ 100 nm in all images.
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0.31 cm3 gꢃ1, respectively, were obtained for this sample. The
values obtained for surface area and pore volume per unit mass
of this sample are large, especially when considering the fact that
the sample has relatively larger mass due to its solid silica core as
well as a heavy metal, Pd. For instance, the surface areas of
mesoporous materials which have a fully porous structure
throughout and contain no heavy metal typically give surface
areas of ꢀ200–1,000 m2/g and pore volumes of ꢀ0.5–1 cm3/g
depending on the type of synthetic method used.36,37 The average
BJH pore diameter calculated from desorption branch of the
nitrogen isotherm for SiO2/Pd–NP/Porous-SiO2 nanospheres
etched for 80 min was found to be 15 nm. Gas adsorption
measurement of SiO2/Pd–NP/Porous-SiO2 nanospheres etched
for slightly longer time of 100 min was also performed
(Figure S5†). An increase in the average BJH pore diameter and
a broader pore size distribution were observed upon increasing
the etching times. For instance, the pore diameters for the sample
etched for 100 min gave a rather broad pore size distribution with
Fig. 4 Plot of conversion of styrene as a function of reaction time in five
successive cycles of reaction using the etched SiO2/Pd–NP/Porous-SiO2
core-shell-shell nanosphere catalyst.
pores predominantly >ꢀ15 nm with pores as large as 50 or 60 nm
in diameter. This was consistent with TEM images (Fig. 2) which
showed a more porous silica shell and slightly bigger pore sizes as
the etching time increases.
The Pd–NP assembled within the SiO2/Pd–NP and SiO2/Pd–
NP/Porous-SiO2 core-shell-shell nanospheres were also charac-
terized by X-ray powder diffraction (XRD) (Figure S6). The
results showed that diffraction intensities of the Pd–NP were
relatively low and had pronounced peak broadening. This is to
be expected for small nanoparticles and more so for those
embedded within silica shell. Nevertheless, the diffraction
patterns clearly indicated the presence of Pd38 and no significant
palladium oxide phase. This, however, does not necessarily rule
out the possibility of the presence of a monolayer of PdO on the
Pd nanoparticles. Further comparison of the diffraction pattern
of Pd–NP in the SiO2/Pd–NP core-shell nanospheres with those
in the SiO2/Pd–NP/Porous-SiO2 core-shell-shell nanospheres
indicated essentially no change in Pd nanocrystal size as calcu-
lated by Scherrer equation. This can also be clearly seen by the
absence of no further broadening of diffraction peaks of the Pd–
NP after the deposition of the silica shell around Pd–NP.
Therefore, it can be concluded from the XRD analysis that the
Pd–NP encapsulated within the nanoporous silica shell remained
stable and are most likely pure Pd and not PdO. The latter was
also supported by the fact that the SiO2/Pd–NP/Porous-SiO2
material showing active catalytic activity in hydrogenation and
C–C coupling reactions; i.e. Pd but not PdO is expected to show
such catalytic activity (see below).
Styrene hydrogenation39–41 was used as a model reaction to
explore the catalytic activity of the encapsulated Pd–NP in the
SiO2/Pd–NP/Porous-SiO2 core-shell-shell nanospheres (Table 1
and Fig. 4). In a control experiment where no Pd–NP was present
(or upon using the silica nanospheres or etched silica nano-
spheres), the hydrogenation reaction did not occur. However,
when SiO2/Pd–NP/Porous-SiO2 core-shell-shell nanospheres
etched for 80 min were introduced into the reaction mixture, the
reaction proceeded even at room temperature and 20 bar H2
pressure giving ethylbenzene as the sole product. Interestingly,
Fig. 3 (A) Nitrogen adsorption–desorption isotherms of SiO2/Pd–NP/
Porous-SiO2 core-shell-shell nanospheres that were synthesized by
etching in aqueous NaOH solution for 80 min and (B) their corre-
sponding pore size distribution.
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Table 1 Hydrogenation of various substrates by SiO2/Pd- NP/Porous-
SiO2 core-shell-shell nanosphere catalystsa
nanoparticles; shaped nanoparticles often give higher catalytic
activities compared to their spherical counterparts;47 and (6) an
easy access of reactants to the surfaces of Pd–NP through the
nanoporous silica shell.
b
c
Substrate Product
t [h]/T [ꢁC] Conv. [%]
Sel. [%] TOF [hꢃ1
]
To study the scope of the catalytic activity of the encapsulated
Pd–NP of the SiO2/Pd–NP/Porous-SiO2 nanospheres, hydroge-
nation reactions of some other compounds were also explored.
Table 1 summarizes the catalytic activity of the porous SiO2/Pd–
NP/Porous-SiO2 nanospheres in the hydrogenation of these
other compounds. For instance, hydrogenation of phenyacety-
lene was achieved efficiently and almost quantitatively in 30 min
using the SiO2/Pd–NP/Porous-SiO2 core-shell-shell nanospheres
as catalyst. Furthermore, the reaction gave selectively only one
product, ethylbenzene. Because the hydrogenation of phenyl-
acetylene can form two possible products: styrene and ethyl-
ebenzene, this reaction is suitable for studying the degree of
catalytic selectivity of hydrogenation. The two-step hydrogena-
tion of phenylacetylene selectively (ꢀ100%) into ethylbenzene
took place only in 30 min with high TOF in the presence of SiO2/
Pd–NP/Porous-SiO2 nanosphere catalyst (Table 1, Entry 2).
Furthermore, effective hydrogenation of cyclohexene to cyclo-
hexane also proceeded well in the presence of SiO2/Pd–NP/
Porous-SiO2 nanospheres, indicating the applicability of the
materials as catalysts for relatively less reactive unsaturated
compounds. In addition, the hydrogenation reaction of a more
stable nitrobenzene into aniline was obtained in 97% yield afteꢁr 3
h reaction time and at a slightly elevated temperature of 50 C
when catalysed by the SiO2/Pd–NP/Porous-SiO2 nanosphere
catalyst. The slightly lower catalytic efficiency of the nanospheres
in hydrogenating nitrobenzene compared to olefins is in line with
the intrinsic lower tendency of nitro groups to undergo hydro-
genation compared to olefins.48 On the other hand, the fact that
the encapsulated Pd–NP in SiO2/Pd–NP/Porous-SiO2 showing
higher catalytic activities for Pd-catalyzed hydrogenation
compared to other materials in similar reactions39,42 could be
attributed to the most conducive structure that they have
including stable and ‘‘naked’’ Pd–NP that are stabilized and
encapsulated within a nanoporous silica shell.
0.5/25
0.5/25
100
100
100
100
5181
5407
1/50
3/50
96
91
100
100
2812
263
a
Reaction conditions: Substrate: 8.73 mmol; methanol: 40 mL; catalyst:
10 mg (3.59 wt.% Pd in the etched SiO2/Pd–NP/Porous-SiO2
nanospheres); 3.9 mol % Pd in the reaction mixture, with respect to
b
mole of reactant and 20 bar H2 pressure. Percent conversion of the
reactant to product(s). In all the cases, only one type of hydrogenated
product was obtained, making the catalytic selectivity 100%.
Selectivity to a particular hydrogenated product.
c
the styrene hydrogenation reached almost ꢀ100% reactant
conversion within 25 min at room temperature. In this catalytic
reaction, the turn-over-frequency (TOF), defined as mol of
styrene converted per total mol of metal per hour, reached 5000
hꢃ1. This high TOF suggests the existence of high catalytic effi-
ciencies for Pd–NP encapsulated within the nanoporous silica
shell in the SiO2/Pd–NP/Porous-SiO2 core-shell-shell nano-
spheres. Comparison of the catalytic efficiencies or TOF values
of our catalyst with respect to other previously reported Pd
catalysts or conventional commercial catalysts was a bit difficult
to do precisely because of the different reaction conditions,
materials and substrates employed in the different cases as well as
the absence of complete catalysis data in some of the previous
reports.39,42–46 However, to get some impression, we had tried to
compare our data with at least those reported complete catalytic
data. For instance, we obtained that the TOF value for hydro-
genation of styrene at ꢀ20 bar H2 pressure and at room
temperature for SiO2/Pd–NP/Porous-SiO2 was three times higher
than that for one of the most efficient polymer supported Pd-
catalysts reported39 for similar reactions at a higher temperature
of 35 ꢁC and 1 atm H2 pressure. In another work,42 a yield of 98%
in 1.5 h for reactant to Pd catalyst ratio of 2 : 1 at room
temperature and 1 atm H2 pressure was reported while in our
case a ꢀ100% yield in 0.5 h for substrate to catalyst ratio of
2000 : 1 at room temperature and 20 bar H2 pressure for a simi-
larly reacting compound was obtained. This means our catalysis
gave a 3000 times greater TOF compared to that in Ref. 42
although a higher hydrogen pressure was employed in our case.
We believe that the high TOF of the catalytic activity of SiO2/
Pd-NP/Porous-SiO2 nanospheres was probably a result of one or
some of the following: (1) the stability (or absence of aggrega-
tion) of the Pd–NP by the virtue of the nanoporous silica shell
around them; (2) the high surface area of the smaller size of Pd
nanoparticles; (3) the presence of stable Pd–NP with naked
surface (or poorly bound surface ligands around them); (4) the
presence of many nanoparticles per each silica nanosphere
support; (5) the faceted nature or octahedral shape of the
The recyclability of the SiO2/Pd–NP/Porous-SiO2 nanospheres
as heterogeneous catalyst was also demonstrated successfully
(Fig. 4). Upon completion of the reaction, the SiO2/Pd–NP/
Porous-SiO2 nanospheres were recovered from the reaction
mixture by simple centrifugation and used in the next reaction
run. The recovered nanospheres showed only a very slight
decrease in their catalytic activity. The slight loss of catalytic
efficiency was most likely due to the loss of catalyst during the
handling and centrifugation. The TEM image (Figure S7†) of the
SiO2/Pd–NP/Porous-SiO2 nanospheres collected after the fifth
successive reaction cycle exhibited no observable change in the
structures of the SiO2/Pd–NP/Porous-SiO2 nanoparticles and
their nanoporous silica shell. The TEM image (Figure S7†),
however, is not high resolution enough to clearly show the Pd–
NP. Nevertheless, based on the absence of significant change in
catalytic activity, we believe that the Pd–NP did not undergo
aggregation to form bulk Pd nor changed their structures and
shapes after the catalytic reactions.
The catalytic reaction of the supernatant of the reaction
mixture after separating the SiO2/Pd–NP/Porous-SiO2 nano-
spheres from it was also run in order to determine whether
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leaching of Pd occurred. We have obtained only 3% styrene
conversion even after 25 min reaction time when the supernatant
was stirred by itself after removing the SiO2/Pd–NP/Porous-SiO2
nanospheres from it. From this result, it is clear that the nano-
porous silica shell has effectively stabilized and prevented the
Pd–NP from aggregation and leaching while at the same time
leaving the Pd nanoparticles’ surfaces somewhat ‘‘naked’’ to give
high catalytic activity.
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Acknowledgements
We gratefully acknowledge the financial assistance by the US
National Science Foundation (NSF), CAREER Grant No.
CHE-064534 and NSF DMR-0804846 for this work.
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