Size-Selective Yolk-Shell Nanoreactors with Nanometer-Thin Porous Polymer Shells

Article abstract of DOI:10.1002/chem.201501968

Yolk-shell nanoreactors with metal nanoparticle core and ultrathin porous polymer shells are effective catalysts for heterogeneous reactions. Polymer shells provide size-selectivity and improved reusability of catalyst. Nanocapsules with single-nanometer porous shells are prepared by vesiclelated directed assembly. Metal nanoparticles are formed either by selective initiation in pre-fabricated nanocapsules or simultaneously with the creation of a crosslinked polymer shell. In this study, we investigated the oxidation of benzyl alcohol and benzaldehyde catalyzed by gold nanoparticles and hydrogenation of cyclohexene catalyzed by platinum nanoparticles. Comparison of newly created nanoreactors with commercially available nanoparticles revealed superior reusability and size selectivity in nanoreactors while showing no negative effect on reaction kinetics.

Full text of DOI:10.1002/chem.201501968

DOI: 10.1002/chem.201501968
Full Paper
&
Heterogeneous Catalysis |Hot Paper|
Size-Selective Yolk-Shell Nanoreactors with Nanometer-Thin
Porous Polymer Shells
Ying Jia,[a] Sergey N. Shmakov,[b] Paul Register,[a] and Eugene Pinkhassik*[b]
Abstract: Yolk-shell nanoreactors with metal nanoparticle
core and ultrathin porous polymer shells are effective cata-
lysts for heterogeneous reactions. Polymer shells provide
size-selectivity and improved reusability of catalyst. Nano-
capsules with single-nanometer porous shells are prepared
by vesicle-templated directed assembly. Metal nanoparticles
are formed either by selective initiation in pre-fabricated
nanocapsules or simultaneously with the creation of a cross-
linked polymer shell. In this study, we investigated the oxida-
tion of benzyl alcohol and benzaldehyde catalyzed by gold
nanoparticles and hydrogenation of cyclohexene catalyzed
by platinum nanoparticles. Comparison of newly created
nanoreactors with commercially available nanoparticles re-
vealed superior reusability and size selectivity in nanoreac-
tors while showing no negative effect on reaction kinetics.
3;5a]
5b,c]
[2l,m]
Silica,[2a–f,n;
carbon,[2g–i;
TiO2,[2j,k] and CeO2
have
Introduction
been the most popular materials used to build shells of NRs.
Encapsulation of a catalyst often dramatically improves its re-
usability. Porous structure of the walls of NRs may offer anoth-
er important advantage over conventional catalysts—the size
selectivity. Indeed, several reports showed that careful control
of pore sizes and their uniformity makes it possible to discrimi-
nate transport of molecules by their size.[2n;3a,f,g;5a] However, for
the vast majority of reported NRs, the size selectivity remains
a challenge and is often achieved at the expense of the diffu-
sion rate or the conversion of substrates due to the thickness
of the shells, which are usually much thicker than tens of
nanometers. Thicker shells are associated with slower diffusion
rates, particularly when well-defined molecular weight cut-off
for permeability is required.[3f; 6]
This work addresses the challenge of creating yolk-shell nano-
reactors combining size selectivity and fast reaction rates.
These nanoreactors contain gold or platinum nanoparticles en-
trapped in hollow porous styrene or acrylate polymer nano-
capsules with single-nanometer shell thickness that enabled
selective permeability and fast mass transfer through the
shells.
Hybrid structures composed of nanosized components en-
capsulated in matrix materials have drawn considerable atten-
tion in the past few years. One particularly promising type of
these nanostructures known under different names—yolk-
shell, core@shell, nanorattle, or ship-in-a-bottle nanoreactors
(NRs)—consists of metal nanoparticles (NPs) confined within
shells or voids of organic or inorganic material.[1] Semiperme-
able walls of NRs act as a physical barrier against large-scale
metal NPs aggregation.
Many applications are not compatible with the typical condi-
tions for the synthesis of NRs, harsh reactions conditions, or
thick inorganic shells. Although polymer shells may offer sub-
stantial advantages in construction and applications of NRs,
they are severely underused. Main challenges include difficul-
ties of producing polymer shells allowing selective ultra-fast
mass transfer of reactants and products while maintaining
structural integrity of NRs.[7a,b] In pursuit of new materials for
the yolk-shell nanoreactors, it is essential to focus on combin-
ing size selectivity with fast mass transfer. Additionally, recycla-
bility is an important consideration for successful practical ap-
plications of nanoreactors.
The combination of active core and porous shells defines
the rapidly developing application of such NRs in heteroge-
neous catalysis.[1] To date, many different reactions were cata-
lyzed by NRs, including reduction,[2] oxidation,[3] halogena-
tion,[2a] and a few others.[4]
[a] Y. Jia, P. Register
Department of Chemistry
Saint Louis University
3501 Laclede Ave, 63103 (USA)
We have recently established two methods of synthesis of
polymer yolk-shell structures with metal cores. These structures
contain single or multiple metal nanoparticles of controlled
size ranging from a single nanometer to tens of nanometers
entrapped within hollow polymer capsules with typical diame-
ters in the 100–200 nm range. The shells of these nanocapsules
have single nanometer thickness and programmed nanopores.
One of the synthesis methods uses vesicle-templated simulta-
[b] S. N. Shmakov, E. Pinkhassik
Department of Chemistry
University of Connecticut
55 North Eagleville Road, Storrs, 06269, CT (USA)
E-mail: eugene.pinkhassik@uconn.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501968.
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neous polymerization of nanocapsules and growth of entrap-
ped NPs.[8a] The size of entrapped NPs can be controlled by
the concentration of metal ion in the aqueous core of the vesi-
cles and the amount of initiator in the bilayer. In another
method, NPs are grown exclusively inside pre-fabricated
hollow polymer nanocapsules.[8b] In the latter approach,
growth of gold NPs is seeded by entrapped sacrificial initiator
molecules. The type of initiator, concentration of metal ions,
and the length of the synthesis control the size of NPs.[8b] Pre-
liminary data showed high activity of such yolk-shell Au NRs in
catalyzing the reduction of 4-nitrophenol.[8b]
Hollow polymer capsules with ultrathin (1–1.5 nm) walls
used here for the construction of yolk-shell NRs above are syn-
thesized by using hydrophobic interior of lipid or surfactant bi-
layers as templating medium.[9a–d,k] Hydrophobic monomers are
placed into the self-assembled bilayers and then polymerized
and crosslinked to create nanocapsule shells. The templating
scaffold can be removed and recycled. This directed assembly
approach allowed us to create a variety of capsule-based nano-
devices.[9e–h] The permeability of the polymer nanocapsule
shells can be controlled by imprinting uniform molecular size
nanopores using different pore-forming templates, as well as
by pore functionalization.[9a,f,i,j] By controlling the size and
chemical environment of nanopores, we were able to achieve
size-selective permeability as well as transport regulated by ex-
ternal environment.[9a,f,i,j] Bilayer-templated porous polymer
walls with single-nanometer thickness enable ultra-fast trans-
port of ions and small molecules while being impermeable to
molecules larger than the pore size.[9b]
Figure 1. TEM (A) and SEM (B) micrographs of a typical polymer NC contain-
ing entrapped Au NP early stage kinetics of methyl benzoate formation. Au-
&
Cat=reference Au@TiO2 (circles, a) and Au NRs ( , c); MeONa, 308C,
O2 3 atm.
into catalytic performance of polymer nanoreactors under oxi-
dative conditions.
This study describes successful use of hollow polymer nano-
capsules in the creation of size-selective and fast-acting nano-
reactors. Here we report catalytic performance, recyclability,
and size selectivity of NRs with Au and Pt movable cores en-
trapped in vesicle-templated capsules with porous nanometer-
thin shells.
To compare activities of Au NRs and reference catalyst, we
studied the early stage kinetics of benzyl alcohol esterification
(Figure 1C; Figures S3 and S4, Supporting Information). At con-
version rates below 10%, the reaction kinetics can be ade-
quately approximated with a linear fit. When two catalysts
were used in amounts corresponding to the same total surface
area of Au NPs, we observed the same reaction rates for Au
NRs and the reference catalyst. This observation suggests that
shells do not slow down the diffusion of reactants and the
products in and out of nanocapsules. The rate-limiting step in
the overall process is the catalytic reaction rather than the
mass-transfer though the shells.
Results and Discussion
By varying reaction conditions (concentration, temperature, or
time), one can control the size of entrapped NPs. In this work,
we used polyether dendrimer DE-(OH)12 as a sacrificial mole-
cule to initiate and grow gold NPs exclusively inside nanocap-
sules (Figure 1 A,B; Figures S1 and S2, Supporting Information).
This approach produced NPs with the average size of 5.4 nm,
as determined from TEM data.[8b] Atomic absorption experi-
ments showed 0.4–0.6 wt.% of gold loading in NRs. The total
surface area of gold NPs was calculated from the average size
of NPs to be approximately 56 m2 gÀ1.
To expand the range of catalytic applications, we prepared
Pt NRs by vesicle templated synthesis using acrylate monomers
(Figure 2A,B; Figure S5, Supporting Information). The growth
of Pt NPs was initiated simultaneously with the formation of
the polymer shell of the nanocapsules, similarly to our previous
report on the creation of silver nanorattles.[8a] Dynamic light
scattering data showed no effect of Pt ions on the formation
of polymer shells (Figure S6, Supporting Information). Reaction
conditions were selected to create Pt NPs with the sizes close
to commercially available Pt NPs supported on activated
carbon (Figures S7 and S8, Supporting Information).
Commercially available Au NPs supported on TiO2 were used
as a reference. They have been extensively studied in alcohol
oxidation and esterification reactions.[10] The total accessible
area of TiO2-supported NPs was calculated according to pub-
lished procedures[11] to be approximately 112 m2 gÀ1. Benzyl al-
cohol esterification is a one-pot cascade reaction in which Au
NPs catalyze both steps (formation of benzaldehyde intermedi-
ate and its conversion to an ester). Studying the behavior of
Au NRs in this process should provide an important insight
Hydrogenation of alkenes by supported Pt or Pd nanoparti-
cles has been widely used as a popular benchmark of catalyst
efficiency due to its simplicity and substantial industrial rele-
vance.[2g,n; 7d]
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Figure 3. Performance of Au NP catalysts in multiple subsequent reactions
measured by the yield of the ester for Au NRs (striped bars) and Au@TiO2
(solid bars).
Figure 2. TEM micrographs of Pt NRs (A) and expanded view of Pt NPs en-
trapped within nanocapsules (B). Early stage kinetics of cyclohexene hydro-
&
*
genation (C). PtCat=reference Pt/C ( , a) and Pt NRs ( , c); 408C; H2
1 atm.
We used catalytic hydrogenation of alkenes to evaluate the
performance of Pt NRs and to compare it with commercially
available Pt NPs supported on activated carbon (Pt/C; Fig-
ure 2C; Figures S9 and S10, Supporting Information). Similarly
to the experiments above, kinetic studies were done using
samples containing the same amount of Pt NPs. Atomic ab-
sorption experiments showed approximately 4.6 wt.% of plati-
num loading in NRs.
Figure 4. Size distribution of Au NPs in Au@TiO2 and Au NRs. Sizes of Au
NPs were measured from TEM micrographs. Samples A and C show Au NPs
in Au@TiO2 before the reaction (A) and after 4th run of esterification reac-
tion (C). Samples B and D show Au NPs in Au NRs before the reaction (B)
and after 4th run of esterification reaction (D).
Again, shells of Pt NRs did not show any negative effect on
the reaction rates, which suggests rapid diffusion of reactants
and the products. No side product formation was detected.
To study reusability of catalysts, we investigated the forma-
tion of methyl benzoate described above. Esterification of
benzyl alcohol proceeds through the formation of benzalde-
hyde as an intermediate in the rate-limiting step (Figures S3
and S4, Supporting Information). To simplify the experiments,
we used benzaldehyde as a substrate for the study of reusabili-
ty of catalysts.
Au NPs from 2.8 nm in starting material to 3.8 nm on average
after the fourth run (Figure 4A,B; Figure S13, Supporting Infor-
mation). This growth alone would result in the decrease of the
total surface area of Au NPs by approximately 27%. In addi-
tion, atomic absorption measurements showed overall Au loss
of 9Æ1.3% after the fourth run.
Combined Ostwald ripening and leaching of gold ions ac-
counts for only a fraction of the activity loss, which strongly
suggests that the surface of Au NPs was partially fouled during
these repeated reactions. In contrast, encapsulated Au NPs nei-
ther changed in size nor leached out of nanocapsules upon re-
peated experiments (Figure 4C,D). Polymer shells likely protect-
ed Au NPs from surface fouling.
In four sets of experiments, Au NRs showed no loss of activi-
ty (Figure 3; Figure S11, Supporting Information). In contrast,
the activity of catalyst supported on TiO2 decreased dramatical-
ly (Figure 3; Figure S12, Supporting Information). Calculated
turnover number, TON, for Au NRs was approximately 2200 for
the four reactions. Under identical conditions, the yield in the
reaction catalyzed by Au@TiO2 dropped by the fourth run from
~71 to ~25%. Calculated TON for four runs was approximately
1600. Apparent reasons for the activity loss could be the Ost-
wald ripening process and leaching of gold ions during the
washing step. TEM images indicate growth of TiO2 supported
Pt NRs showed similar repeatable performance. In six subse-
quent experiments, we found no evidence of the loss of activi-
ty (Figures S14 and S15, Supporting Information).
As shown previously, the size of intrinsic pores in the wall of
vesicle-templated nanocapsules is less than 0.8 nm.[9a–c] To
probe the size-selective permeation through the walls of nano-
capsule, we compared the conversion of benzaldehyde and its
sterically crowded analogue, 3,5-di-tert-butyl benzaldehyde
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the same reaction conditions as described above. After each
reaction step, nanocapsules were redispersed in water. During
these steps, any released TSPPNa molecules would be separat-
ed from the nanocapsules. The smallest cross-section of
TSPPNa is approximately 1.5 nm. No escape of fluorescent
TSPPNa molecules was observed under reaction conditions
used here. Retention of the originally encapsulated TSPPNa
(Figure 6) suggests that the capsules did not break or develop
pinhole defects.
Conclusion
In summary, encapsulation of metal NPs in hollow nanocon-
tainers with porous polymer walls with single-nanometer thick-
ness offers size selectivity and improved catalyst recycling
without compromising reaction kinetics. Au NRs maintain the
same level of catalytic activity and withstand relatively harsh
reaction conditions (high concentration of a base, oxidizing
agent, high pressure, etc.) in repeated catalytic reactions, while
the activity of reference catalyst degrades significantly. Nano-
reactors based on encapsulated metal NPs proved to be effi-
cient catalysts for different types of reactions. As shown previ-
ously, the permeability of the polymer nanocapsule shell can
be controlled by imprinting uniform molecular size nanopores
using different pore-forming templates, as well as by pore
functionalization.[9f,i,j] Considering straightforward control of
size and chemical environment of nanopores,[9a,f,i,j] we antici-
pate that ultrathin polymer-based NRs will be easily expanded
to different types of catalysts and catalytic processes.
Figure 5. Size-selective oxidation of benzaldehyde and DTBBA by Au@TiO2
(solid bars) and Au NRs (striped bars). Conversion of DTBBA required
a longer reaction time than benzaldehyde. For each substrate, the reaction
was carried out by using the amount of catalyst corresponding to the same
total surface area of gold.
(DTBBA), on Au NRs and on Au@TiO2. Smallest cross-sections
of benzaldehyde and DTBBA are 0.6 and 0.9 nm, respectively
(Figure 5). Conversion of DTBBA required a longer reaction
time. Figure 5 clearly shows size selectivity of Au NRs, as com-
pared with no selectivity by TiO2-supported catalyst (Fig-
ure S16, Supporting Information). This remarkable size selectivi-
ty is achieved without compromising the mass-transfer rate
through the shell.
To test the durability of ultrathin polymer shells we designed
control experiments simulating reaction conditions. SEM analy-
sis and permeability studies with nanocapsules containing en-
trapped dyes confirmed that the structural integrity of NRs re-
mained intact under both oxidative and reductive reaction
conditions (Figure 6). In these experiments, tetrasodium-meso-
tetra(4-sulfonatophenyl) porphine (TSPPNa) was entrapped in
polymer nanocapsules. Samples were repeatedly subjected to
Experimental Section
General
Chemicals were purchased from Sigma–Aldrich unless noted other-
wise. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) was pur-
chased from Avanti Polar Lipids, Inc. as a dry powder. Gold 1% on
TiO2 (Au@TiO2) was purchased from STREM Chemicals. 3,5-Di-tert-
butylbenzaldehyde (DTBBA) was purchased from Frontier Scientific.
All chemicals were used as received, unless noted otherwise. Butyl
methacrylate (BMA), tert-butyl methacrylate (tBMA), tert-butylstyr-
ene (tBuSt), ethylene glycol dimethacrylate (EGDMA), and p-divinyl-
benzene (DVB), were passed through an alumina column to
remove the inhibitor shortly before the polymerization. Solvents
(Fisher Scientific) were HPLC-grade and used as received. All glass-
ware was washed with aqua regia prior to synthesis of gold nano-
particles. Polyether dendrimer DE-(OH)12 was prepared as described
previously (Figure S2, Supporting Information).[8b]
Methods
Figure 6. A) SEM micrograph of Au NRs after 4th run in benzyl alcohol
esterification confirming preservation of spherical shape of nanocapsules;
B) Retention of fluorescent molecules supporting lack of degradation of cap-
sule shells: aqueous dispersions of nanocapsules with entrapped water-solu-
ble fluorescent tetrasodium-meso-tetra(4-sulfonatophenyl) porphine
(TSPPNa) molecules (smallest cross-section 1.5 nm): control sample (1), nano-
capsules containing Au NPs and TSPPNa after 4th run in benzyl alcohol
esterification (2), nanocapsules containing Pt NPs and TSPPNa after 4th run
in 1-hexene reduction (3). Lack of release of TSPPNa from samples 2 and 3
supports structural integrity of nanocapsule shells under the reaction condi-
tions.
Shimadzu GC 2010-Plus equipped with FID and Shimadzu GCMS
2010 were used to monitor reaction kinetics. Hydrodynamic diame-
ter measurements were performed on a Malvern Nano-ZS Zetasizer
(Malvern Instruments Ltd., Worcestershire, U.K.). SEM and Scanning
Transmission Electron Microscopy (STEM) images were acquired on
FEI Inspect F50 microscope. TEM was carried out on FEI Tecnai
Spirit and JEOL JEM-2010 FasTEM microscopes. Atomic absorption
experiments were carried out on GBC 908AA instrument equipped
with a graphite furnace.
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Synthesis of styrene nanocapsules
Size of Au and Pt nanoparticles
Nanocapsules were prepared using a previously described proce-
dure.[9c] Two monomers solutions containing (S1) Cetyltrimethylam-
monium tosylate (CTAT) and (S2) sodium dodecylbenzenesulfonate
(SDBS) were prepared. Solution S1: tBuSt (105 mL, 0.57 mmol), DVB
(81 mL, 0.57 mmol), 2,2-dimethoxy-2-phenyl-acetophenone, DMPA,
(3 mg, 0.01 mmol), and CTAT (200 mg, 0.438 mmol) were added to
a test tube, followed by addition of 2.5 mm DE-OH12 aqueous solu-
tion (20 mL). Solution S2: tBuSt (105 mL, 0.57 mmol), DVB (81 mL,
0.57 mmol), DMPA, (3 mg, 0.01 mmol), and SDBS (200 mg,
0.57 mmol) were added to a test tube, followed by addition of
2.5 mm DE-OH12 aqueous solution (20 mL). Solutions were kept at
35–408C for 15 min, then shaken or briefly sonicated to give ho-
mogeneous dispersion. Then solutions were quickly mixed up in
the desired proportion. In a typical experiment, solutions were
mixed in the ratio S1/S2=2:8 (i.e., 2 mL of S1 and 8 mL of S2),
shaken several times and then bathed undisturbed for 30 min at
358C. The resulting suspension containing vesicles of different
sizes was extruded 4–5 times at 358C through a track-etched poly-
ester membrane with 200 nm pore size. Oxygen was removed by
passing argon through the solution. The sample was irradiated
(l=254 nm) in a photochemical reactor equipped with a stirrer (10
lamps of 32 W each; 10 cm distance between the lamps and the
sample) for 60 min. Methanol (10 mL) was added, and the precipi-
tate was washed 3–5 times with methanol and then 3 times with
acetonitrile over a period of 24 h.
The size of Au and Pt nanoparticles was determined from TEM mi-
crographs. Size distributions were calculated by measuring more
than 300 nanoparticles from a collection of several micrographs. In
these measurements, several micrographs were taken from
random areas of the sample. Each nanoparticle within each of
these micrographs was measured and the results were plotted on
a histogram.
Dye loading and retention experiments
Dye loading and retention experiments were performed according
to a previously described procedure.[8b] Polymer nanocapsules with
entrapped tetrasodium-meso-tetra(4-sulfonatophenyl) porphine
(TSPPNa) were prepared by following the same procedures de-
scribed above for synthesis of liposome-templated nanocapsules
except for the hydration step, in which 2.5 mm aqueous solution
of TSPPNa was used instead of solution of DE-(OH)12. After dye en-
capsulation, nanocapsules were thoroughly washed with methanol
and finally with water until complete disappearance of the Soret
band at 416 nm in the UV/Vis spectra of the supernatant was ob-
served. Nanocapsules were subjected to the same reaction condi-
tions followed by addition of water, precipitation, and recording of
the UV spectrum of the supernatant.
Catalysis
All experiments were carried out in a heavy-walled glass reactor
equipped with magnetic stir bar. For kinetic studies, the corre-
sponding reaction was stopped and an aliquot was tested by GC
analysis.
Au-loaded polymer nanoreactors (Au NRs)
A slurry of precipitated nanocapsules (ca. 5 mg of dry material)
was transferred to a screw-capped test tube equipped with a stir
bar and dispersed in 2 mL of acetonitrile. The test tube was placed
in water bath and stirred at 608C. Then 100 mm HAuCl4 aqueous
solution (10 mL) was added to nanocapsules dispersion and the
mixture was agitated for 5 min at 608C. Then 1m NaOH (10 mL)
was added. After 15 min, the mixture was removed from the bath
and ice-cold water (8 mL) was added. The resulting colored precipi-
tate was washed with water and methanol.
Esterification of benzyl alcohol
Benzyl alcohol (10.4 uL) was added to
a reactor containing
25 wt.% sodium methoxide solution (2.7 g) in methanol, methanol
(7.5 mL), powdered Au@TiO2 (2.6 mg, ~0.13 mol%). tert-Butylben-
zene or mesitylene were used as an internal standard. The reaction
was carried out under 3 atm. of oxygen at 308C.
Pt-loaded polymer nanoreactors (Pt NRs)
Esterification of benzaldehyde and DTBBA
Pt NRs were prepared by a method similar to the liposome-tem-
plated method described earlier for silver loaded nanocapsules.[8a]
In a typical experiment, tBMA (43 mL, 0.193 mmol), BMA (42 mL,
0.199 mmol), EGDMA (32 mL, 0.17 mmol), and DMPA, (1 mg, 3.9”
10À6 mol) were added to a solution of DMPC (0.4 mL, 160 mg,
0.236 mmol) in CHCl3. The CHCl3 was evaporated using a stream of
argon to form a lipid/monomer mixture on the wall of a test tube.
The film was further dried under vacuum for 5 min to remove
traces of CHCl3. The dried film was hydrated with 10 mm aqueous
solution of neutralized H2PtCl6 (8 mL) to give a dispersion of multi-
lamellar vesicles. During the hydration of the lipid/monomer mix-
ture, the test tube was briefly agitated on a Vortexer every 5 min.
The suspension was extruded 16 times at 358C through a track-
etched polyester Nucleopore membrane (Sterlytech) with 0.2 mm
pore size using a Lipex stainless steel extruder (Northern Lipids).
Prior to polymerization, unloaded platinum ions were removed
from the mixture by size-exclusion chromatography on Sephadex
G-50 column. Oxygen was removed by passing argon through the
solution. Polymerization was carried out as described above. Meth-
anol (10 mL) was added, and the precipitate was washed 3–5 times
with methanol and then 3 times with ethanol.
Benzaldehyde (10.1 uL, 0.1 mmol) or 3,5-ditertbutylbenzaldehyde
(21.8 mg, 0.1 mmol) was added to a reactor containing 25 wt.%
sodium methoxide solution (2.7 g) in methanol, methanol (7.5 mL),
powdered Au@TiO2 (2.6 mg, ~0.13 mol%). tert-Butylbenzene or
mesitylene were used as an internal standard. The reaction was
carried out at 308C under 3 atm. of oxygen.
Catalytic reactions with Au NRs
Catalytic reactions with Au NRs were carried out in the same way
as described above, but instead of Au@TiO2, the precipitate of Au
NRs was used. Calculated total accessible surface area of Au nano-
particles and Au@TiO2 were equivalent.
Hydrogenation of cyclohexene with Pt/C
Cyclohexene (100 uL) was added to a reactor containing Pt/C
(0.25 mg, 0.013 mol% of 10 wt.% Pt/C) in ethanol (4.4 mL) at 408C
under 1 atm. of hydrogen.
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We thank R. Ristau for help with TEM imaging. This work was
supported by the National Science Foundation (CHE-1012951,
CHE-1316680, and CHE-1522525).
Keywords: gold nanoparticles · heterogeneous catalysis · ship-
in-a-bottle
·
size-selective nanocapsules
·
yolk-shell
nanoreactors
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Received: May 19, 2015
Published online on July 28, 2015
Chem. Eur. J. 2015, 21, 12709 – 12714
www.chemeurj.org
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