Dendritic porous yolk@ordered mesoporous shell structured heterogeneous nanocatalysts with enhanced stability

Article abstract of DOI:10.1039/c7ta07271f

One of the major challenges in heterogeneous catalysis is how to suppress the aggregation of thermodynamically unstable noble metal nanoparticles (NPs) and simultaneously maintain their high accessibility. Here we report the fabrication of integrated functional nanostructures consisting of a dendritic porous silica yolk with many small noble metal NPs and a protective mesoporous silica shell (MSS) with perpendicularly aligned pore channels and tunable shell thickness by using a well-controlled interfacial engineering strategy. Three-dimensional (3D) dendrimer-like superstructures with many permeable center-radial large pore channels and a highly accessible internal surface area, named dendritic porous silica spheres (DPSSs), serve as unique yolks not only because of their capability to accommodate high density ultrafine Au or Pt NPs, but also their functionality to act as robust physical barriers to separate and confine the aforementioned loaded NPs, which result in slowing down their aggregation at high temperatures due to Ostwald ripening. These integrated hierarchical structures also ensure good stability under weak basic and acidic conditions. Due to their superior structural properties, the DPSSs@noble metal NPs@MSS yolk-shell structures exhibit excellent catalytic performance in p-nitrophenol reduction, epoxidation reaction and CO oxidation. The favorable stability and high catalytic performance of these yolk-shell structures make the developed design strategy very useful for the fabrication of novel highly active and stable catalysts.

Full text of DOI:10.1039/c7ta07271f

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Dendritic porous yolk@ordered mesoporous shell structured
heterogeneous nanocatalysts with enhanced stability
Xin Du,*ab Caixia Zhao,a Yi Luan,*a Changbin Zhang,c Mietek Jaroniec,d Hongwei Huang,e Xueji
Zhanga and Shi-Zhang Qiao*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
One of the major challenges in heterogeneous catalysis is how to suppress the aggregation of thermodynamically unstable
noble metal nanoparticles (NPs) and simultaneously maintain their high accessibility. Here we report the fabrication of
integrated functional nanostructures consisting of a dendritic porous silica yolk with many small noble metal NPs and a
protective mesoporous silica shell (MSS) with perpendicularly aligned pore channels and tunable shell thickness by using a
well-controlled interfacial engineering strategy. Three-dimensional (3D) dendrimer-like superstructures with many
permeable center-radial large pore channels and highly accessible internal surface area, named as dendritic porous silica
spheres (DPSSs), serve as unique yolks not only because of their capability to accommodate high density ultrafine Au or Pt
NPs, but also their functionality to act as robust physical barriers to separate and confine the aforementioned loaded NPs,
which result in slowing down their aggregation at high temperatures due to the Ostwald ripening. These integrated
hierarchical structures also ensure good stability under weak basic and acidic conditions. Due to the superior structural
properties, the DPSSs@noble metal NPs@MSS yolk-shell structures exhibit excellent catalytic performance in p-
nitrophenol reduction, epoxidation reaction and CO oxidation. The favorable stability and high catalytic performance of
these yolk-shell structures make the developed design strategy very useful for the fabrication of novel highly active and
stable catalysts.
www.rsc.org/
high accessible surface areas.[4,5] Calcination of nanocatalysts
at high temperatures is a common method to activate and
regenerate them respectively before catalytic reactions and
1. Introduction
Ultrafine noble metal nanoparticles (NPs, especially < 10 nm)
with high stability, tunable particle sizes, shapes and
composition are of considerable significance for the
development of highly active, selective and recyclable
heterogeneous catalysts.[1-3] Unfortunately, free noble metal
NPs (especially Au) have a large tendency to grow and
aggregate during high temperature processing and catalytic
reactions, which is mainly driven by van der Waals interactions
and high surface energy owing to their ultrasmall size and the
presence of numerous surface unsaturated coordination
sites.[4,5] The unexpected and irreversible aggregation leads to
a distinct loss of catalytic activity. To circumvent this problem,
noble metal NPs are usually loaded in/on suitable carriers with
after recycling tests; these processes involve the removal of
organic capping agents and adsorbed reactant/product organic
molecules from the surface of noble metal NPs. However, even
though stable supports are employed, the thermodynamically
unstable noble metal NPs are prone to deactivation due to
their migration on the support surface and subsequent
agglomeration by particle coalescence during high
temperature reactions, which is ascribed to high surface
energies and low Tammann temperatures.[4,5]
Up to now, to prevent noble metal NPs from
sintering/coalescing, various strategies have been developed
to immobilize NPs on/in novel structured solid supports.[6-18]
However, most often these strategies inevitably cause the loss
of the exposed surface active sites due to the complete or
partial embedding of noble metal NPs in carrier matrix.
Recently, yolk-shell (also named as rattle-type) structured
nanomaterials with a permeable mesoporous shell have been
recognized as very promising nanoreactors.[19-22] As compared
with porous, core-shell, hollow or partially embedded
composite nanocatalysts, the yolk-shell structures have unique
features such as:[19-22] (1) permeable channel-type mesopores
in the shells allowing small guest molecules to quickly diffuse
into the hollow cavity and subsequently interact with the
movable functional yolks; (2) hollow cavities between shells
a.Research Center for Bioengineering and Sensing Technology, School of Chemistry
& Biological Engineering, Beijing Key Laboratory for Bioengineering and Sensing
Technology, University of Science & Technology Beijing, Beijing 100083, P. R.
China. E-mail: duxin@ustb.edu.cn, yiluan@ustb.edu.cn
b.School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005,
Australia. E-mail: s.qiao@adelaide.edu.au
c. Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences,
Beijing 100085, China
d.Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio
44242, USA
e. School of Materials Science and Technology, China University of Geosciences,
Beijing 100083, China
Electronic Supplementary
DOI: 10.1039/x0xx00000x
Information
(ESI)
available:
See
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and yolks providing large storage space to achieve the high hydrochloric acid (HCl, 37 wt. %) were purchased from Beihua
loading capacity of guest molecules; (3) moveable functional Fine Chemicals. Ultrapure water with a resistivity higher than
yolks without capping agents featuring a large number of 18.2 MΩ·cm was used in all experiments and was obtained
surface active sites assuring high performance; and (4) more from a three-stage Millipore Mill-Q Plus 185 purification
importantly, protective shells suppressing irreversible system (Academic).
aggregation and leaching of functional yolks. Such structures
2.2. Synthesis of DPSSs
are more efficient from the economical and structural
Monodispersed DPSSs with center-radial large pores were
viewpoints than those containing only hollow cavity with one
prepared using CTA·Tos as the surfactant, [BMIM] OTF ionic
movable core. Although the concept of multiple cores with
liquid as a cosurfactant, TEA as the mineralizing agent, water
ultrasmall sizes encapsulated in one hollow cavity is better,
as solvent and TEOS as silicon source according to the reported
another issue emerges how to suppress aggregation of
method.[23] A typical synthesis was performed as follows: a
multiple cores in one hollow cavity at high temperatures.
mixture of CTA·Tos (0.96 g), [BMIM] OTF (0.3 g), TEA (0.105 g)
Therefore, the smart design and controllable synthesis of novel
and water (50 mL) was stirred at 80 °C for 1 h, and then TEOS
catalytic systems capable to overcome the abovementioned
(7.29 g) was quickly added into the surfactant solution. The
issues is highly desirable.
final mixture was stirred at 80 °C with a stirring speed of 1000
In this work, we report the well-controlled interfacial
rpm for another 2 h. The synthesized DPSSs were centrifuged,
engineering method for the synthesis of unique dendritic
washed, and dried in the oven at 60 °C. The CTA·Tos and
porous yolk@mesoporous silica shell (MSS) structured noble
[BMIM] OTF were removed from particles by template
metal heterogeneous nanocatalysts with high stability, which
extraction. The as-prepared DPSSs (1.0 g) were added into
are composed of three parts: (1) MSS with perpendicularly
ethanolic HCl (conc. HCl (15 mL) in ethanol (100 mL)), and
aligned pore channels for protecting the active metal NPs and
sonicated for 2 h. The suspension was stirred at 70 °C for 24 h.
shortening diffusion paths as compared with disordered pore
The extraction procedure was repeated twice to efficiently
channels; (2) rigid dendritic porous silica spheres (DPSSs) as
remove the surfactant. Finally, the precipitates were
movable yolks for loading many small noble metal NPs,
centrifuged, washed with pure water, and dispersed in water
assuring their full accessibility and stability; (3) numerous small
or dried in air at 60 °C.
noble metal NPs assuring efficient catalytic reactions. To the
2.3. Aminopropyl-functionalization of DPSSs
best of our knowledge, this is the first report on the use of
dendritic porous yolks loaded with multiple small active metal
NPs. The resulting structures possess high density small noble
metal (Au: 2~12 nm or Pt: < 4 nm) NPs in the center-radial
pores of DPSSs and hollow cavity between DPSSs and MSS,
assuring high specific surface area of 682 m2·g-1. Catalytic
testing showed their excellent catalytic performance. In
addition to facilitate the fast transport rate of reaction
substrates and product molecules through two permeable
pore channels, the integrated yolk-shell nanostructures with
open dendritic porous yolk and perpendicularly aligned
mesoporous shell can also suppress thermally induced
aggregation of noble metal (Au or Pt) NPs, and maintain the
structural stability under weak basic and acidic conditions.
The aminopropyl-functionalization of the pore walls of DPSSs
was carried out by post-grafting method. In
a typical
procedure, 1.0 g of DPSSs were dispersed in 80 mL of toluene
in a two-round bottom flask, and the mixture was treated by
ultrasonic dispersion at ultrasonic power of 300 w for 30 min.
After the mixture was stirred at 80 oC for 30 min, 0.5 mL of
o
APTES was added and continued to stir at 80 C for 12 h. The
aminopropyl-functionalized DPSSs (DPSSs-NH2) were obtained
by washing with ethanol for five times.
2.4. Loading noble metal NPs on DPSSs-NH2
100 mg of DPSSs-NH2 were dispersed in 20 mL of water, and
sonicated at ultrasonic power of 300 w for 30 min. After
adding 0.1 mL of HAuCl4·4H2O (H2PtCl6·6H2O) solution (100
mg·mL-1) the mixture was magnetically stirred in a closed flask
for 2 h at room temperature, an excessive amount of fresh
aqueous NaBH4 (10 mM) was added into the above mixture
until the colour turned red. The precipitate (DPSSs-NH2@Au or
Pt NPs) was centrifuged, washed with water, and dried in air at
60 oC. It is worth noting that the above solution was colourless
after centrifugation, suggesting that nearly all the HAuCl4
(H2PtCl6) was reduced into Au (Pt) NPs, which were all
immobilized into the pores of DPSSs-NH2 to form DPSSs-
NH2@Au (Pt) NPs composites.
2. Experimental
2.1. Materials
Tetraethyl
aminopropyltriethoxysilane
cetyltrimethylammonium tosylate (CTA·Tos,
orthosilicate
(TEOS,
≥
99
%),
98
3-
(APTES,
≥
%),
≥
98 %),
cetyltrimethylammonium bromide (CTAB, ≥ 99 %), 1-butyl-3-
methylimidazolium trifluoromethanesulfonate ([BMIM] OTF)
and triethanolamine (TEA, ≥ 99 %) were purchased from
Sigma Aldrich. Resorcinol (≥ 99%), formaldehyde solution (37
wt. %), chloroauric acid (HAuCl4·4H2O, ≥ 99 %), chloroplatinic
acid (H2PtCl6·6H2O, ≥ 99 %), sodium borohydride (NaBH4, ≥
99%), aqueous ammonia (NH3·H2O, 25~28 wt. %), absolute
ethanol (≥ 99.5 %), toluene (≥ 99.9 %), sodium hydroxide (≥
2.5. Coating RF polymer resin on DPSSs-NH2@Au (Pt) NPs
The synthesis of RF polymer resin was performed as reported
elsewhere,[24] in which resorcinol and formaldehyde were
mixed with water and ethanol and used as the precursors of RF
resin, while NH3·H2O was used to initiate the polymerization
99.9 %)
, 4-nitrophenol (≥ 99.5 %) and concentrated
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process. In a typical coating process, 100 mg of DPSSs-NH2@Au NPs@MSS (35 nm) was used in the catalytic reaction for 10
(Pt) NPs were dispersed in the mixed solvent of 20 mL of successive cycles under identical conditions, and the yellow
ethanol and 10 mL of water, and the mixture was treated by suspension became colorless within 8 min every time.
ultrasonic dispersion at ultrasonic power of 300 w for 30 min.
HCl treatment: DPSSs@Au NPs@MSS (shell thickness: 35 nm)
In order to regulate the thickness of RF layer, the varied and DPSSs-NH2@Au NPs were dispersed in HCl solution (pH = 2)
amount of resorcinol (0.15, 0.2 or 0.3 g) and aqueous and stirred for 20 min, respectively. After that, the catalysts
ammonia (0.60 mL) were added and the resulting mixture was were centrifuged and washed with water until pH=7.
stirred for 20 min at 30 oC. Then formaldehyde solution (0.225, Subsequently, their UV-vis absorption spectra were recorded.
0.3 or 0.45 mL) was added, and the system was kept under Finally, the above-mentioned reduction reaction of 4-NP was
magnetic stirring for 24 hours. DPSSs-NH2@Au (Pt) NPs@RF carried out to investigate the catalytic performance.
composites were finally collected by centrifugation and
2.9. Catalytic oxidation of cyclooctene and styrene
washed with water three times.
Cyclooctene and styrene epoxidation reaction over the catlayst
2.6. Synthesis of DPSSs-NH2@Au (Pt) NPs@RF@ MSS
was carreied out at 1 atm pressure. A mixture of 1 mol% of
The formation of mesoporous silica shell was performed via a gold nanoparticle catalyst, 1.0 mmol of olefin and 1.5 mL of
surfactant-templating sol–gel strategy by using CTAB as a acetonitrile was stirred for 30 min at room temperature. After
template. In a typical synthesis, 37.5 mg of CTAB was dissolved adding 0.4 mL of t-butyl hydroperoxide (3.0 equivlent, 70 wt%
in the solution containing 25 mL of deionized water and 13 mL in H2O) into the above mixture, the reaction vessel was
of ethanol. After stirring at 30 oC for 15 min, 1 mL of an immersed in an oil bath, which was heated to 80 ºC. After
ethanol dispersion solution containing 50 mg of DPSSs- certain time, a minor amount of the solution was taken for
NH2@Au (Pt) NPs@RF composites was added. The mixture was conversion and selectivity determination through gas
homogeneously dispersed by ultrasonication for 30 min, charomatography. The catalytic epoxidation reaction generally
followed by the addition of 0.25 mL of NH3·H2O. After stirring takes 12 h total reaction time.
for additional 10 min, a solution containing 75 μL of TEOS and
2.10. Catalytic oxidation of CO
1.0 mL of ethanol was added dropwise under continuous
o
stirring for 1 min. After the mixture was reacted at 30 C for 6
Catalytic activities for CO oxidation were monitored in a fixed-
bed flow reactor. A reaction gas (CO/O2/N2 = 1 : 20 : 79) was
h, the products were centrifuged, washed three times with
passed through a catalyst bed (50 mg of DPSSs@Pt NPs (size:
water and once with absolute ethanol, and then dried at 60 oC
ca. 1~4 nm)@MSS (shell thickness: ca. 30 nm) (Pt content: ca.
overnight, after which the core–shell-shell DPSSs-NH2@Au (Pt)
2.1 wt. %), which was pelletized, and then sieved by mesh with
NPs@RF@MSS spheres were obtained. To fabricate thicker
pore size of 0.250~0.425 mm) at a flow rate of 100 mL min-1
MSS, 55 (75) mg of CTAB and 110 (150) μL of TEOS were used
(space velocity = 120 000 mL·gcat−1·h−1). The effluent gas
while other experimental conditions were unchanged.
stream was monitored by on-line FTIR (Nicolet iS50) equipped
2.7. Fabrication of DPSSs@Au (Pt) NPs@MSS
with 2 m gas cell and a DTGS detector. The conversion of CO
Removal of the RF polymer resin, CTAB surfactant and other was calculated from the change of CO concentration at the
organic species was performed by calcination of DPSSs- reactor inlet and outlet:
NH2@Au (Pt) NPs@RF@MSS spheres in air from room
temperature to 550 oC for 6 h at a rate of 1 oC·min-1, after
which the yolk-shell DPSSs@Au (Pt) NPs@MSS spheres were
obtained.
CO conversion = ([CO]in – [CO]out)/[CO]in
2.11. Characterization
Scanning electron microscopy (SEM) observations were carried
out on Hitachi S-8010 scanning electron microscope
operated at 10 kV. Specimens were coated with a layer of gold
×100%
a
2.8. Catalytic reduction of 4-nitrophenol
The catalytic properties of DPSSs@Au NPs@MSS with different by ion sputtering before SEM observations. Energy dispersive
shell thickness of 35, 60 and 95 nm were investigated via the X-ray (EDX) spectroscopy was performed on a Hitachi S-4800
reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as scanning electron microscope at an acceleration voltage of 150
a model reaction with NaBH4 as a reductant. Aqueous solution kV. For transmission electron microscopy (TEM) observations,
of 4-NP (1 mL, 2.5 mM), a fresh aqueous NaBH4 (2 mL, 250 the samples dispersed in ethanol were added on super-thin
mM) and 3 mL of water were added into a glass bottle under carbon film-coated copper grids, dried and observed on a JEOL
stirring at room temperature. After 10 min, 0.1 mL of aqueous JEM-2100F transmission electron microscope at an
solution of DPSSs@Au NPs@MSS (5 mg/mL) was added. As the acceleration voltage of 200 kV. X-Ray powder diffraction (XRD)
reaction proceeded, the bright yellow solution gradually faded. patterns of products were recorded on a Bruker D8 Focus X-
The reaction progress was monitored by UV-vis absorption ray diffractometer using Cu Ka radiation (λ = 0.154184 nm),
spectroscopy, which was achieved by removing a small portion and used to identify their phase constitutions and crystallite
(0.2 mL) of the reaction mixture after given periods of time sizes. The zeta-potentials of various nanoparticles were
and adding it to water (3 mL) for the immediate determination measured by using a Malvern Zetasizer Nano ZS90 (Malvern
of the UV-Vis absorption spectra. The catalysts were Inst. Ltd., U.K.) equipped with four-side clear cuvettes or the
immediately separated after reaction by centrifugation at 9500 ZET 5104 cell at room temperature. UV-vis absorption spectra
rpm for 2 min and reused in the next cycle. DPSSs@Au in the wavelength range of 200~600 nm were recorded by
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using
a
Shimadzu
UV-1800
spectrophotometer. highly accessible internal surface area as compared to
Thermogravimetric analysis (TGA) was performed on a SDT conventional ordered mesoporous silica spheres with uniform
Q600 thermogravimetric analyzer (TA Instruments) at
pore sizes.[25] The modification of pore walls of DPSSs with
a
heating rate of 10 oC·min-1 under a flow of air gas. Fourier aminopropyl groups assures high dispersion and large loading
transform infrared (FTIR) spectra were recorded on a Varian of active noble metal (Au or Pt) NPs,[26,27] and the subsequent
Excalibur 3100 spectrometer. The Au contents in the deposition and growth of resorcinol-formaldehyde (RF)
DPSSs@Au NPs@MSS samples were measured with an Optima polymer resin on the surface of DPSSs via an interfacial
5300DV inductively coupled plasma atomic emission polymerization is used to create later a free space between
spectroscopy (ICP-AES) (Perkin–Elmer). Before each ICP-AES yolk and mesoporous shell.[28,29] Then, MSS with tunable shell
measurement, the silica component in the product was first thickness and perpendicularly aligned mesopore channels is
dissolved with NaOH solution, and then dropwise added in formed on the surface of RF layer via an organic-inorganic self-
excessive aqua regia to dissolve the Au component, and finally assembly process.[30-32] Such dendritic porous yolk@noble
solution pH was kept at 2~3. Nitrogen adsorption–desorption metal (Au or Pt) NPs@MSS nanomaterials are finally subjected
measurements were carried out on an automated surface area to calcination in air under high temperature to remove all
and pore size analyzer (Quantachrome (USA), QuadraSorb SI) organic species.
at -196 oC using the volumetric method. The products were
SEM and TEM images are used to monitor every stage of the
dried at 120 oC before analysis. Pore size distributions were synthesis process. As displayed in Fig. 2a,b and S1a-d in
estimated from adsorption branches of the isotherms by using Supporting Information, the obtained DPSSs are composed of
the Barrett, Joyner, and Halenda (BJH) method or Quenched wrinkled nanosheets (thickness: ca. 10~24 (15±3) nm) being
Solid Density Functional Theory (QSDFT) method. Brunauer– pore walls of the radial channels, which are arranged in three
Emmett–Teller (BET) specific surface areas were calculated by dimensions to form uniform spheres with particle sizes of
using adsorption data in the P/P0 range of 0.05~0.25. Pore 243±26 nm. The formed center-radial large mesopores and
volumes were determined from the amounts of N2 adsorbed at macropores have irregular slit-like porous structure with wide
the single point of P/P0 = 0.98.
surface pore size distribution of 57±31 nm (Fig. S2). Nitrogen
adsorption-desorption measurements show that DPSSs have a
high BET surface area of 282 m2·g-1 and a total pore volume of
0.93 cm3·g-1 (Fig. S1e,f). After modification of DPSSs with
aminopropyl groups, the zeta potential changed from −25.5
mV (DPSSs) to +34.5 mV (DPSSs-NH2) (Fig. S3a,b), indicating
successful functionalization. Then, Au NPs were loaded on the
DPSSs-NH2 structures by in-situ reduction of HAuCl4 with
NaBH4.[26,27] SEM and TEM images in Fig. 2c,d and S4 show high
density of Au NPs with small sizes of 1.73±0.35 nm well-
dispersed in the center-radial pores of DPSSs-NH2, suggesting
the uniform distribution of aminopropyl groups on the pore
walls.[26,27] The XRD pattern of DPSSs-NH2@Au NPs (Fig. 3a)
3. Results and Discussion
3.1. Material Synthesis and Characterization
Schematic illustration of synthetic strategy of yolk-shell
structured DPSSs@noble metal NPs@MSS nanocatalysts is
shown in Fig. 1. DPSSs are used as novel yolks due to their
unique open three-dimensional (3D) dendrimer-like
superstructure with center-radial large pore channels and
shows one wide Au diffraction peak at 2
suggests ultrasmall size of Au NPs, as well as a very broad
signal at 2
= ca. 22.3o, which is ascribed to amorphous silica.
θ
= ca. 38.1o, which
θ
The loading of well-dispersed Au NPs causes a decrease of zeta
potential from +34.5 mV (DPSSs-NH2) to +19.1 mV (DPSSs-
NH2@Au NPs) (Fig. S3c). The residual surface positive charge
favors heterogeneous interfacial growth of RF polymer resin
with negative surface charge on the surface of DPSSs-NH2@Au
NPs due to the charge compatibility.[28,29] The thickness of RF
layer can be tuned from ca. 15 to 45, then to ca. 70 nm by
controlling the initial amount of added resorcinol and
formaldehyde (Fig. 2e and S5). In addition, during the
interfacial growth of RF layer, the size of Au NPs increases
from 1.73±0.35 nm to ca. 4.2±1.7 (1~6) nm. The possible
reason of the aggregation of Au NPs may be that the abundant
phenolic groups of the gradually formed RF resin layer provide
strong multidentate coordination interactions with the
ultrasmall Au NPs formed by chemical reduction.[33,34] And
the dynamic formation and growth process of RF resin layer on
the heterogeneous interface of DPSSs-NH2@Au NPs may drive
adjacent ultrasmall Au NPs to grow into a bit larger Au NPs.
Fig. 1. Schematic illustration of the fabrication of unique dendritic porous
yolk@noble metal NPs@MSS structured nanocatalysts, which are
composed of: (1) MSS with perpendicularly aligned pore channels for
protecting the active metal NPs and shortening diffusion paths as
compared with disordered pore channels; (2) rigid DPSSs as movable yolks
for loading many small noble metal NPs, maintaining their accessibility and
stability; (3) many small naked noble metal NPs (Au or Pt) for efficient
catalytic reactions.
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However, the exact reason is still very unclear. The XRD resulting spheres in air at 550 oC for 6 h, the color of the
patterns of DPSSs-NH2@Au NPs@RF (Fig. 3a) exhibit four Au product changes from brown (DPSSs-NH2@Au NPs@RF@MSS)
diffraction peaks, which are assigned to the (111), (200), (220), to light red (DPSSs@Au NPs@MSS) (Fig. S7). TGA curve of
and (311) reflections of the cubic (fcc) gold lattice (JCPDS DPSSs-NH2@Au NPs@RF (shell thickness: ca. 45 nm)@MSS
cards: No. 04-0784), respectively, indicating an enhancement shows a weight loss of ca. 52.9 % in the temperature range
in the degree of Au crystallization as compared with DPSSs- from 100 to 700 oC, which should be attributed to the removal
NH2@Au NPs. The formation of RF layer also causes of organic species including the RF layer, CTAB and
aggregation among DPSSs-NH2@Au NPs@RF, which increases aminopropyl groups (Fig. S8). FTIR spectra reveal that all
with increasing thickness of the RF layer (Fig. S5). The organic species were completely removed (Fig. S9), suggesting
aggregation should largely result from the consecutive that the surface of Au NPs is naked. The structures of both
dynamic growth of RF resin layer on the surface of DPSSs- MSS with perpendicularly aligned mesopore channels and
NH2@Au NPs combined with the unavoidable collisions among DPSSs with dendritic pores are well-maintained (TEM images in
initial DPSSs-NH2@Au NPs and then among DPSSs-NH2@Au Fig. 2g,h), while the cavity between MSS and DPSSs is not
NPs@RF. It is expected to reduce the aggregation by clearly visible, which results from a drastic shrinkage of MSS
elaborately tuning the experimental parameters in the next toward particle interior due to the removal of RF layer.[35] In
study.
addition, the thickness of MSS also shrinks from ca. 40 nm to
ca. 35 nm owing to high temperature calcination, while the
size of Au NPs increases from ca. 4.2±1.7 (1~6) to 5.6±3.5
(2~12) nm, which is in a good agreement with that calculated
from the XRD pattern based on Scherrer equation (Fig. 3a).
Although obvious thermally induced aggregation of Au NPs
occurs, Au NPs still have high particle density of ca. 80~150 Au
NPs per one DPSS@Au NPs@MSS, and most of them is still
present in the center-radial pores of DPSSs and a few appear in
the space between MSS and DPSSs (TEM images in Fig. 2g,h).
The surface area and pore structure of DPSSs@Au NPs@MSS
were
characterized
by
N2
adsorption-desorption
measurements. The obtained isotherm exhibits a well-defined
hysteresis loop at higher relative pressures (P/P0 = 0.45~0.95)
(inset in Fig. 3b), suggesting the presence of mesopores. The
pore size distribution curve (Fig. 3b) shows one narrow strong
peak at 3.3 nm due to uniform mesopores in MSS and the
other wide peak (5~40 nm) centered at 9~17 nm due to the
presence of center-radial pores in DPSSs and hollow cavities
between MSS and DPSSs.
Fig. 3. (a) XRD patterns of DPSSs-NH2@Au NPs, DPSSs-NH2@Au NPs@RF
and DPSSs@Au NPs@MSS. (b) N2 adsorption-desorption isotherm (inset)
and the corresponding pore size distribution curve obtained for DPSSs@Au
NPs@MSS (35 nm).
Fig. 2. SEM (a) and TEM (b-i) images of DPSSs (a,b), DPSSs-NH2@Au NPs
(c,d), DPSSs-NH2@Au NPs@RF (e), DPSSs-NH2@Au NPs@RF@MSS (f) and
DPSSs@Au NPs@MSS (g,h).
In addition, the thickness of MSS can be easily tuned from
ca. 35 to 60, then to ca. 95 nm by increasing the amount of
TEOS added (Fig. 4a-e). When the thickness of MSS is ca. 95
nm, numerous small Au NPs (2~8 nm) are present in the DPSS
channels, and the distance between MSS and DPSS is ca. 12
nm (Fig. 4c and S10), which may suggest that such a thick MSS
Subsequently, MSS was grown on the RF surface layer via a
surfactant (CTAB)-directed sol-gel interfacial self-assembly
process.[30-32] The formed MSS has perpendicularly aligned
mesopore channels with pore sizes of ca. 3~4 nm, and its
thickness is ca. 40 nm (Fig. 2f and S6). After calcination of the
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slightly inhibit the drastic shrinkage of MSS towards particle hollow cavities, i.e., resulting in the formation of only one or
interior. Moreover, DPSSs@Pt NPs@MSS with tunable MSS several large Au NPs. However, complete or partial
a
thickness of ca. 30, 50 and 80 nm were also fabricated by embedding of noble metal NPs in matrix can markedly
similar synthesis strategy (Fig. 4f-h). The ultrasmall (size: < 4 suppress thermally induced aggregation,[11-14] but it can
nm) Pt NPs can also be loaded in DPSSs with high density of > diminish the exposed active surface of noble metal NPs,
100 Pt NPs per one DPSS@Pt NPs@MSS. The sizes (size: < 4 nm) reducing the overall catalytic activity. In comparison, the
of Pt NPs are much smaller than those (2~12 nm) of Au NPs, unique yolk-shell structures containing dendritic particles as
which may due to both smaller loading of NPs (ca. 2.1 wt. % vs. yolks still maintain high density of over 80 Au NPs (size:
3.4 wt. %) and higher Tammann temperature. All these 5.6±3.5 (2~12) nm) per one DPSSs@Au NPs@MSS (Fig. 4a-e),
DPSSs@Au (Pt) NPs@MSS have MSS with perpendicularly thus significantly inhibiting sintering of Au NPs. Even though
o
aligned mesopore channels and DPSSs with open center-radial DPSSs@Au NPs@MSS were calcined at 550 C for 6 h, nearly
large mesopores, which are in favor of the transport of no obvious change was observed in the hierarchical structure
reactants/products through two permeable pore channels and and Au NPs (Fig. S12). This may be due to that the
subsequent contact with active noble metal NPs. These results interconnected wrinkled nanosheets of dendritic yolk with
indicate that the synthesis strategy described here to fabricate excellent structural rigidity and thermal stability,[41,42] can
DPSSs@noble metal NPs@MSS with unique yolk-shell function as robust physical barriers to separate and confine Au
structures is effective and reliable. This design presents the NPs, thus efficiently slowing down the aggregation degree of
first combination of dendritic yolks and mesoporous shells, noble metal NPs. In order to prove it, DPSSs-NH2@Au NPs
assuring several structural advantages and consequently, an without MSS were directly calcined at 550 oC for 6 h to
excellent catalytic performance.
investigate their stability. TEM images in Fig. S13 show that 3D
dendritic porous structure of DPSSs is well-maintained, and a
high density (> 100 Au NPs per one DPSS) of small (2~8
(5.4±1.4) nm) homogeneously dispersed Au NPs in DPSSs is
visible, indicating the structural stability of DPSSs at 550 oC and
their role in inhibiting aggregation of Au NPs as compared with
particle-surface loaded-type nanocatalysts.[11-14] However,
several large Au NPs appear on DPSSs due to the lack of MSS
(Fig. S13a). After further treatment at elevated temperature
(650 oC for 6 h), a lot of somewhat larger Au NPs are still
uniformly loaded in DPSSs, but dendritic porous structure of
DPSSs shows a severe shrinkage (Fig. S14), which blocks the
molecular transport through pore channels. Thus, 550 oC
calcination is better for removal of organic species.
3.3. Catalytic reactions
To evaluate the catalytic performance of DPSSs@many small
noble metal NPs@MSS structures, a typical catalytic reaction,
the liquid-phase reduction of 4-nitrophenol (4-NP) to 4-
aminophenol (4-AP) with NaBH4, was chosen (Table S1). The
catalytic reduction of harmful 4-NP into less toxic and reusable
4-AP via noble metal-based nanocatalysts is regarded as an
efficient, green, and economical method for environmental
treatment. The color of the 4-NP solution varied from light
yellow to light bright yellow after adding NaBH4 into the
aqueous solution of 4-NP (Fig. S15a), and concomitantly the
UV-vis absorption peak red-shifts from 317 to 400 nm (Fig.
S15b) due to the deprotonation of 4-NP in alkaline (pH: 8~9)
solution. The redox reaction did not occur in the absence of
catalyst even after 2 days, which was confirmed by the
unchanged absorption peak at 400 nm. After the introduction
of a small amount of DPSSs@Au NPs@MSS catalyst, the yellow
color of the reaction mixture gradually faded as the reaction
proceeded (Fig. S15a). Fig. S15c shows the time-dependent
UV-vis absorption spectra for the reduction of 4-NP in the
presence of DPSSs@Au NPs@MSS. The absorption peak at 400
nm gradually became weaker and simultaneously absorption
peak at ca. 298 nm gradually became stronger with reaction
Fig. 4. SEM (a) and TEM (b-h) images of DPSSs@Au NPs@MSS with
different shell thickness size: 95 (a-c), 60 (d) and 35 nm (e), and DPSSs@Pt
NPs@MSS with varied shell thickness size: 80 (f), 50 (g) and 30 nm (h).
3.2. Thermal stability
For most hollow or yolk-shell structures containing a few or
many Au NPs (Fig. S11),[36-40] high temperature calcination (550
oC) causes a drastic thermally induced aggregation of Au NPs in
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time, demonstrating the reduction of 4-NP and formation of presence of numerous easily accessible small noble metal NPs.
the deprotonated 4-AP, respectively. The catalytic reaction Although the highest catalytic activity was not achieved,
shows that 100 % of conversion was achieved within 8 min by DPSSs@noble metal NPs@MSS have unique structural
using DPSSs@Au NPs@MSS (35 nm) with a shell thickness of advantages of resisting thermally- (Fig. 4,S12)- or base/acid-
-
ca. 35 nm. Since the added BH4 is excessive, the redox induced aggregation of Au NPs.
reaction can be considered to be a pseudo-first-order one. Fig.
Next, the recyclability of DPSSs@Au NPs@MSS (35 nm) was
5a exhibits the plots of ln(Ct/C0) vs. reaction time, where Ct and further investigated. The recovered catalyst still displayed a
C0 are the concentrations of the deprotonated 4-NP at time t high catalytic performance with a conversion over 98 % within
and 0, which can be measured from the relative intensity of 8 min even after 10 successive reuse cycles (Fig. 5b), indicating
the absorbance At and A0, respectively. The rate constant k that the catalytic stability of DPSSs@Au NPs@MSS is relatively
values for 4-NP reduction determined from the slopes were high. TEM images (Fig. S16) of DPSSs@Au NPs@MSS after 10
calculated to be 7.7×10-3, 4.5×10-3 and 0.95×10-3 s-1 for cycles show that the structure of both DPSSs and MSS remain
DPSSs@Au NPs@MSS with different shell thickness of ca. 35, almost unchanged, but a slight aggregation of Au NPs from
60 and 95 nm, respectively (Fig. 5a). Furthermore, the 5.8±3.5 to 6.4±4.9 nm is visible. These results indicate that the
corresponding molar rate constants were estimated to be ca. weak basic (8~9) conditions caused by NaBH4 cannot destroy
8.92×104, 5.72×104 and 1.50×104 s-1·mol-1, respectively, based the hierarchical structures even after 10 reuse cycles of
on the molar Au content in DPSSs@Au NPs@MSS with varied reaction. In addition, many practical catalytic reactions have to
shell thickness, which were measured to be ca. 3.4, 3.1 and 2.5 proceed in acidic conditions or involve acidic reactants,[43] thus
wt. % by ICP-AES analysis. These results indicate that a it is highly desirable to achieve the high stability of
decrease in the thickness of MSS can help to improve the nanocatalysts in such acid conditions. The acidic stability of
catalytic efficiency, which should mainly result from the DPSSs@Au NPs@MSS (35 nm) and DPSSs-NH2@Au NPs was
shorter pore channels facilitating the transport of reaction investigated (Fig. 5c,d). DPSSs-NH2@Au NPs initially featured
substrates and product molecules.[32] Furthermore, to further much higher rate constant k than DPSSs@Au NPs@MSS (35
reveal the intrinsic activity, the turnover frequency (TOF) of nm) due to open center-radial large pores assuring fast mass
DPSSs@Au NPs@MSS (35 nm) was calculated to be ca. 3.62 transport, and higher loading and smaller sizes of Au NPs.
min-1 by the mole of the reduced 4-NP/the mole of Au However, after treatatment with HCl solution, the rate
content/the consumed time. This TOF is higher than those of constant k of DPSSs-NH2@Au NPs decreased remarkably from
most Au NPs-loaded nanocatalysts (Table S1), but a little bit 19.3×10-3 to 4.1×10-3 s-1 as compared with that of DPSSs@Au
lower than those of a small number of nanocatalysts without NPs@MSS (35 nm), which just showed a slight decrease from
the protective mesoporous shell. The high catalytic 7.7×10-3 to 6.0×10-3 s-1. This behavior is mainly ascribed to
performance is attributed to the integrated structure: acid-induced Au NPs aggregation in DPSSs-NH2@Au NPs, which
perpendicularly aligned pore channels in MSS and open is proved by UV-vis absorption spectra. The absorption peak of
dendritic pores in DPSSs facilitating mass transport, and to the
DPSSs-NH2@Au NPs shows a distinct red-shift from ca. 514 to
534 nm after HCl treatment due to Au NPs aggregation (Fig.
S17a), which can be caused by protonation of -NH2 group able
to break the coordination bond between the empty d orbital
of Au atom and the free electron pair on the N atom of -NH2
group. In addition, for DPSSs@Au NPs obtained after
o
calcination at 550 C for 6 h, a visible red-shift from ca. 530 to
540 nm also appears (Fig. S17b), indicating that the loading of
Au NPs on DPSSs is not firm enough to endure this HCl
treatment. In contrast, the UV-vis absorption peak of
DPSSs@Au NPs@MSS (35 nm) only shows a minute change
from 530 to 531 nm (Fig. S17c), suggesting that MSS combined
with dendritic silica yolk can suppress the acid-induced
aggregation of Au NPs (i.e., assure high stability under acid
conditions).
Catalytic epoxidation reaction is a highly practical organic
transformation for the rapid synthesis of important epoxide
intermediates.[44] The epoxidation of cyclooctene was first
evaluated by using the optimal DPSSs@Au NPs@MSS (35 nm)
catalyst. Control reaction shows the lack of conversion without
Au catalyst in the presence of tert-butyl hydroperoxide (TBHP).
As can be seen in Fig. 6a DPSSs@Au NPs@MSS exhibited a
great conversion of cyclooctene under the optimal reaction
Fig. 5. (a) Plots of ln(Ct/C0) versus the reaction time for the catalytic
reduction of 4-NP by NaBH4 without or with DPSSs@Au NPs@MSS
nanocatalyst. (b) Repeated catalytic tests of the DPSSs@Au NPs@thin MSS
for the reduction of 4-NP. (c) Schematic illustration of DPSSs@Au
NPs@MSS (35 nm) and DPSSs-NH2@Au NPs. (d) Comparison of catalytic
activity of DPSSs@Au NPs@MSS and DPSSs-NH2@Au NPs after their
treatment with HCl solution (pH = 2) for 20 min.
conditions. The selectivity of cyclooctene oxide
2 was
determined to be 82 %. Meanwhile, the selectivities of enone
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3
and enol
4
were measured to be 6 % and 5 %, respectively. the catalytic activity of Pt NPs due to the strong electron
catalyst for cyclooctene interaction between Pt and CO. The oxidation reaction of CO
When HAuCl4 was used as
a
epoxidation, it showed poor performance (<10 % cyclooctene was carried out in a fixed-bed flow reactor at a high space
conversion) in terms of conversion and selectivity. Note that velocity of 120 000 mL·gcat−1·h−1 (short contact time of the
this selectivity is higher than many values reported in reactant flow with catalyst). Fig. 7 shows that the CO
literature.[45,46] while the conversion remains to be great. conversion as
a function of reaction temperature over
Furthermore, the epoxidation of styrene was investigated to DPSSs@Pt NPs (size: ca. 1~4 nm)@MSS (shell thickness: ca. 30
understand the product distribution vs. reaction temperature nm) (TEM in Fig. 4h), which has the thinnest shell, thus should
and time (Fig. 6b-d), which reveals the selectivity of kinetically- have the optimal performance as proved by the 4-NP
controleld product and thermodynamically-controlled product reduction reaction. Under such high space velocity of 120 000
during reaction.[47-49] As the temperature increased from 40 to mL·gcat−1·h−1, the 100 % CO conversion was achieved at the
o
80 ºC, the initial reaction rate increased, while the selectivity temperature of ca. 175 C, which is lower than those of most
of the desired styrene oxide decreased (Fig. 6c). At 40 ºC for 6 Pt@silica nanocatalysts reported elsewhere.[11,51-56] This
h, both conversion (95 %) and selectivity (91 %) reached the superior catalytic performance may be ascribed to the high
maximum (Fig. 6d). After 6 h, the selectivity declined as the pore permeability of hierarchical structures and high density of
conversion was still maintained at 99 %, which is due to the ultrasmall naked Pt NPs (size: ca. 1~4 nm).
formation of thermodynamically-controlled benzaldehyde
product. In comparison to the literature data,[30,31,50] styrene
epoxidation reaction under our conditions displayed the
improved conversion and selectivity. The results suggest that
the epoxidation performance of supported gold catalysts is
highly related to the nature of support and the size of Au NPs.
A plausible reaction mechanism has been proposed for the
styrene epoxidation reaction, as exhibited in Fig. S18. The
reaction proceeds through the formation of hydroperoxy
species, which deliver the oxygen to the double bond of the
styrene molecule. After styrene oxide
6
is generated, it can be
. In addition, it
partially converted to phenylacetaldehyde
7
also can be further oxidized to benzaldehyde by an epoxide
ring opening followed by the oxidative cleavage of the C-C
bond. This proposed mechanism can explain why the
selectivity of styrene epoxide decreases distinctly after
achieving the optimal time.
Fig. 7. CO oxidation activity of DPSSs@Pt NPs (size: ca. 1~4 nm)@MSS (shell
thickness: ca. 30 nm) (Pt content: ca. 2.1 wt. %), which was pelletized, and
then sieved by mesh with pore size of 0.250~0.425 mm), at a space velocity
of 120 000 mL·gcat−1·h−1
.
4. Conclusions
In summary, we have successfully fabricated novel
nanostructured catalysts containing a dendritic yolk, loaded
with many small naked noble metal NPs, and having a
protective MSS with tunable thickness via a well-controlled
interfacial engineering strategy. The unique dendritic porous
silica yolk with excellent structural rigidity and thermal stability
has many center-radial large pores composed of wrinkled
nanosheets, which can function as robust physical barriers to
separate and confine the loaded noble metal NPs, thus
efficiently slow down the rate of their aggregation under high
temperature. The unique yolk-shell architecture with open
dendritic porous yolk and perpendicularly aligned mesoporous
shell endows catalysts with several structural advantages: the
fast transport of reaction substrates and product molecules
through permeable pore channels, thermally-induced
aggregation resistance, and the stability of composite catalysts
under weak basic and acidic conditions. Due to these structural
characteristics, DPSSs@noble metal NPs@MSS catalysts show
excellent catalytic performance in 4-NP reduction, epoxidation
Fig. 6. (a) Cyclooctene and (b) Styrene epoxidation by using DPSSs@Au
NPs@MSS (35 nm) catalyst. (c) Conversion and selectivity versus
temperature for styrene epoxidation at the reaction time of 12 h. (d)
Conversion and selectivity versus time for styrene epoxidation at 40 ºC.
To better show the structural advantages of the catalysts
studied, CO molecule was employed as a probe to indentify
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This work was supported by National Natural Science
Foundation of China (Grant No. 21501009, 51503016,
21576202), Beijing Natural Science Foundation (Grant No.
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA07271F
The table of contents entry
Title: Dendritic porous yolk@ordered mesoporous shell structured heterogeneous
nanocatalysts with enhanced stability
X. Du,* C. Zhao, Y. Luan,* C. Zhang, M. Jaroniec, H. Huang, X. Zhang, S. Z. Qiao*
Keywords: yolk-shell structure; dendritic porous silica yolk; mesoporous silica shell; nanocatalysts;
structure stability
The integrated dendritic porous yolk@ordered mesoporous shell structured heterogeneous
nanocatalysts exhibit enhanced stability and superior catalytic performance.
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