Well-Defined Metal Nanoparticles@Covalent Organic Framework Yolk–Shell Nanocages by ZIF-8 Template as Catalytic Nanoreactors

Article abstract of DOI:10.1002/smll.201804419

Yolk–shell nanoreactors have received considerable interest for use in catalysis. However, the controlled synthesis of continuous crystalline shells without imperfections or cracks remains challenging. Here, a strategy for the synthesis of yolk–shell meta

Full text of DOI:10.1002/smll.201804419

FULL PAPER
Covalent Organic Framework Nanoreactors
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Well-Defined Metal Nanoparticles@Covalent Organic
Framework Yolk–Shell Nanocages by ZIF-8 Template
as Catalytic Nanoreactors
Kaixun Cui, Wanfu Zhong, Lingyun Li, Zanyong Zhuang, Liuyi Li,* Jinhong Bi,
and Yan Yu*
while protecting them from aggregation is
highly desirable.
Yolk–shell nanoreactors have received considerable interest for use in
catalysis. However, the controlled synthesis of continuous crystalline shells
without imperfections or cracks remains challenging. Here, a strategy for the
synthesis of yolk–shell metal nanoparticles@covalent organic framework
(MNPs@COF) nanoreactors by using MNPs@ZIF-8 core–shell nanostruc-
tures as a self-template is designed and developed. The COF shell is formed
through an amorphous-to-crystalline transformation process of a polyimine
shell in a mildly acidic solution, while the ZIF-8 is etched in situ, generating
a void space between the MNPs core and the COF shell. With the protection
of the COF shell, multiple ligand-free MNPs are confined inside of the hollow
nanocages. Importantly, the synthetic strategy can be generalized to engineer
the functions and properties of the designed yolk–shell nanocages by varying
the structure of the COF shell and/or the composition of the core MNPs.
Representative Pd@H-TpPa yolk–shell nanocages with active Pd NP cores
and permeable TpPa shells exhibit high catalytic activity and stability in the
reduction of 4-nitrophenol by NaBH4 at room temperature.
Yolk–shell nanostructures, which are
composed of MNPs cores within a hollow
cavity surrounded by an outer shell,[3]
offer great potential to fulfill the optimal
balance between the catalytic activity and
stability of MNPs,[4] in which the MNPs
cores with ligand-free surfaces provide the
active sites for catalytic reaction, while the
shell serves as a barrier to prevent aggre-
gation of the MNPs.[5] To obtain a highly
efficient and stable yolk–shell catalyst, a
shell with structural stability and perme-
ability is of critical importance.[6] The
shells in most earlier reports were amor-
phous.[7] In fact, the crystallinity of the
shell has a substantial effect on the cata-
lytic performance of yolk–shell nanocom-
posites.[8] Previously reported yolk–shell
MNPs@MOF composites have illustrated
the successful utility of crystalline shells
with ordered porosity for heterogeneous catalysis.[9] However,
the conglomerate of the nanocrystals of MOFs is the primary
limiting factor for the formation of a well-packed and con-
tinuous shell in yolk–shell MNPs@MOF nanostructures,[10]
as imperfections or cracks will result in undesirable diffusion
pathways and/or MNP leaching. Moreover, the MOF shells
in most previous yolk–shell MNPs@MOF nanostructures are
microporous.[9a,d] The small pore size would limit the diffu-
sion and mass transport efficiency of large molecules and thus
hinder their access to the internal active sites. Therefore, the
rational design of hierarchical porous crystalline shells hosting
uncapped MNPs is a crucial issue for the development of yolk–
shell nanocatalysts.
1. Introduction
Metal nanoparticles (MNPs) are of fundamental interest owing
to their remarkable catalytic properties. The dispersibility and
accessibility of the MNPs are paramount for their catalytic per-
formance.[1] However, MNPs without surface capping agents
easily aggregate due to their high surface energy, leading to
an unexpected decrease of their catalytic activity over time.[2]
Thus, fully exploiting the intrinsic catalytic properties of MNPs
K. Cui, W. Zhong, Prof. L. Li, Prof. Z. Zhuang, Prof. L. Li, Prof. Y. Yu
College of Materials Science and Engineering
Fuzhou University
New Campus, Minhou, Fujian 350108, China
E-mail: lyli@fzu.edu.cn; yuyan@fzu.edu.cn
Covalent organic frameworks (COFs),[11] known as crystal-
line porous networks,[12] show great potential as carriers for
metal ions or MNPs due to their well-defined tunable struc-
tures.[13] Considering the unique characteristics of COFs, such
as their high porosity, easy modification, and well-defined and
designable topologies,[14] the encapsulation of MNPs inside
the cavities of hollow COFs could be a promising strategy for
developing stable and active yolk–shell catalysts. The sizes
and shapes associated with the chemical and physical proper-
ties of the presynthesized MNPs can be feasibly tuned by their
well-established syntheses, while the adequate coordination
K. Cui, W. Zhong, Prof. L. Li, Prof. Z. Zhuang, Prof. L. Li,
Prof. J. Bi, Prof. Y. Yu
Key Laboratory of Eco-Materials Advanced Technology
(Fuzhou University)
Fujian Province University
Fuzhou, Fujian 350108, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201804419.
DOI: 10.1002/smll.201804419
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Scheme 1. Schematic illustration of the encapsulation of MNPs in a COF nanocage.
interactions between the COF shell and MNP core may not
only stabilize the MNPs in a highly dispersed fashion but
also enhance the catalytic activity by providing a synergistic
effect to the metal active sites. Moreover, the hierarchical pore
structures within the COFs allow for efficient mass transport
through their thin shells. Accordingly, superior catalytic perfor-
mances for the yolk–shell MNPs@COF composites would be
expected.
MNPs@COF yolk–shell cages using MNPs@ZIF-8 core–shell
nanostructures as a self-template (Scheme 1). The ZIF-8 serves
not only as a protector to prevent interparticle aggregation
during the reaction but also as a sacrificial template to gen-
erate the void space between the MNPs core and the COF shell.
The COF shell effectively prevents MNPs aggregation, while the
short and open channels in COF shells provide excellent acces-
sibility to the MNPs by the reactants. Pd@H-TpPa, as a model
yolk–shell catalyst, shows excellent catalytic activity and recy-
cling stability in the hydrogenation of 4-nitrophenol reactions.
Herein, we demonstrate a general method for the encapsu-
lation of multiple MNPs inside the cavities of a COF to form
Figure 1. TEM images of the a) Pd NPs, b) Pd@ZIF-8, c) Pd@ZIF-8@Am-TpPa, and d,e) Pd@H-TpPa; f) HRTEM image of the cubic Pd NPs in
Pd@H-TpPa; g) EDX elemental mapping of Pd@H-TpPa.
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The survey X-ray photoelectron spectroscopy (XPS) spectrum
shows that C, N, O, and Pd elements coexist in Pd@H-TpPa
(Figure S7, Supporting Information), which is consistent with
the EDX results. Pd@H-TpPa and TpPa showed only one
peak, located at 399.7 eV, ascribed to the nitrogen atoms in
the linkage (Figure 2c), indicating either that the N groups do
not interact with the Pd surface or that their interaction does
not modify the electronic properties of N atoms. XPS investi-
gations with Ar-ion etching were conducted on Pd@H-TpPa
(Figure 2d). The results show that the characteristic peaks of
Pd 3d5/2 and Pd 3d3/2 at 335.7 and 340.9 eV[13b] emerged after
Ar+ etching, further confirming the presence of Pd NP cores in
the nanocages. Compared to those of PVP-covered Pd NPs, the
Pd 3d signals of Pd@H-TpPa had shifted 1.0 eV toward higher
binding energies because of the interactions between the car-
bonyl group of the TpPa shell and the Pd NP surface.[20] The
Brunauer–Emmett–Teller (BET) surface area of Pd@H-TpPa
is 42 m2 g−1 (Figure S8, Supporting Information). The lower
surface area than that of TpPa is mainly ascribed to the encap-
sulation of the Pd cores and the relatively weaker crystallinity.
Thermogravimetric analysis (TGA) revealed that the thermal
stability of Pd@H-TpPa is similar to that of pure TpPa
(Figure S9, Supporting Information).
With the protection of the ZIF-8 shell, no aggregation or
morphological variation of the Pd NPs was observed after
encapsulation in the hollow nanocages, which is critical to
maintaining their intrinsic properties. The effect of the heat
treatment on the agglomeration of Pd NP inside the COF
cages was further examined. As shown in TEM images, after
heating of Pd@H-TpPa in water (Figure S10, Supporting Infor-
mation) or under solvent-free (Figure S11, Supporting Infor-
mation) conditions at 100 °C for 6 h, Pd NPs inside the cages
remained their original size, morphology, and distribution after
heat treatment, verifying that the COF shell can effectively pre-
vent agglomeration of metal NPs inside the cages. ZIF-8 was
chosen as the sacrificial template, not only because ZIF-8 can
be etched away during the formation of the COFs in situ under
mildly acidic conditions but also because the affinity of ZIF-8
for the organic precursor assists in the coating of the polyimine
network. Moreover, methods for coating ZIF-8 on different
MNPs have been well developed,[21] which makes our synthetic
strategy general and versatile. Thus, the synthetic strategy
using ZIF-8 as a template can be extended to the synthesis of
other nanostructures with different metal NPs. By using core–
shell Au@ZIF-8 and Ag@ZIF-8 as precursors, Au NPs and Ag
NPs can be embedded into the COF cages (Figure 3), respec-
tively, which further proves the feasibility and generality of our
synthetic strategy for fabrication of MNPs@COF yolk–shell
nanostructures.
2. Results and Discussion
The synthetic strategy of MNPs incorporated into COF cages is
shown in Scheme 1. In a typical synthesis, Pd cubes with edge
sizes of 10 nm (Figure 1a) were first coated with ZIF-8 to form a
Pd@ZIF-8 core–shell nanostructure (Figure 1b and Figure S1,
Supporting Information), which was then mixed with 1,3,5-tri-
formylphloroglucinol (Tp) and p-phenylenediamine (Pa) to
form an amorphous polyimine-network shell by a Schiff-base
reaction outside of the Pd@ZIF-8 (defined as Pd@ZIF-8@
Am-TpPa) (Figure 1c). The subsequent removal of ZIF-8 in a
mixture of 1,4-dioxane and aqueous acetic acid, generating a
void between the Pd NPs and the shell, resulted in the forma-
tion of Pd@H-TpPa with a yolk–shell nanostructure (Figure 1d).
The TpPa shell preserved the well-defined rhombic dodecahe-
dral shape of Pd@ZIF-8. The continuous and smooth shell of
Pd@ZIF-8@Am-TpPa evolved into a rough surface containing
many small grains of Pd@H-TpPa (Figures S2 and S3, Sup-
porting Information). Multiple Pd NPs were well dispersed in
the cages (Figure 1e), and almost no unencapsulated particles
were observed. A high-resolution transmission electron micros-
copy (HRTEM) image shows the presence of lattice fringes
with a d-spacing value of 0.195 nm corresponding to the (200)
plane of Pd (Figure 1f), although they appear indistinct owing
to the coating of the COF shell. Energy-dispersive X-ray (EDX)
elemental mapping of Pd@H-TpPa further verified that Pd NPs
were well distributed in the cavity (Figure 1g). Notably, the shell
thickness, void size, and number of Pd NPs in Pd@H-TpPa can
be facilely tailored by varying the TpPa and ZIF-8 precursors as
well as the concentration of the Pd NPs (Figure S4, Supporting
Information).
Under the acidic conditions used, the amorphous polyimine
shell spontaneously transforms into a crystalline framework
(Figure 2a).[15] The common powder X-ray diffraction (PXRD)
patterns of Pd NPs[16] and ZIF-8[17] were both observed in
Pd@ZIF-8@Am-TpPa. However, no characteristic peaks cor-
responding to crystalline TpPa were found in Pd@ZIF-8@
Am-TpPa, suggesting an amorphous polyimine shell. Upon
exposure to acidic conditions, the peaks at 4.7° (100 plane)
and 27° (001 plane), which were attributed to crystalline TpPa,
emerged,[18] while the characteristic PXRD peaks of ZIF-8
disappeared, indicating a successful disorder-to-order transfor-
mation of the polyimine network and the simultaneously selec-
tive and complete removal of the ZIF-8 template. The weak
crystallinity of TpPa in Pd@H-TpPa was mainly ascribed to the
ambient synthetic conditions that were used.[15] The crystal-
linity of the TpPa shell could be slightly improved by the further
solvothermal treatment of Pd@H-TpPa (Figure S5, Supporting
Information).
−1

The appearance of the C C peak at 1578 cm in the Fou-
Integrating the unique properties of COFs and MNPs, the
MNPs@COF yolk–shell nanostructures would presumably
offer a number of advantages to heterogeneous catalysis. The
encapsulated MNPs serve as the catalytic sites, which can be
facilely tailored to meet the requirements of specific catalytic
reactions by varying the type, shape, size, and quantity of the
MNPs. COF shells with high crystallinity and stability serve as
a host to protect the MNPs against aggregation. In addition, the
periodic pores in the COF shell and the internal void space of
the yolk–shell nanostructures would enhance the mass transfer
rier transform infrared (FTIR) spectra of Pd@ZIF-8@Am-TpPa
and Pd@H-TpPa reveals the formation of the keto–enamine
group in the polyimine shell (Figure S6, Supporting Informa-
tion).[19] The spectrum obtained for Pd@H-TpPa clearly shows
the complete disappearance of the characteristic peaks of ZIF-8,
indicating the complete removal of ZIF-8. The solid-state 13C
NMR spectrum of Pd@H-TpPa indicates the presence of all
of the characteristic peaks of TpPa,[18] verifying the structural
integrity of TpPa during the removal of ZIF-8 (Figure 2b).
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Figure 2. a) PXRD patterns of ZIF-8, Pd NPs, Pd@ZIF-8@am-TpPa, TpPa, and Pd@H-TpPa; b) solid-state 13C NMR of Pd@H-TpPa and TpPa;
c,d) XPS spectra of N 1s and Pd 3d for Pd@H-TpPa.
of the chemical species. Pd@H-TpPa was employed for the
catalytic reduction of 4-nitrophenol by NaBH4 at room tempera-
ture. The reduction kinetics were monitored by UV–vis absorp-
tion spectroscopy of the reaction mixture. No reaction occurred
in the absence of Pd@H-TpPa or when only using TpPa
(Figure 4a, entry 1 in Table 1). The reaction could be efficiently
catalyzed by Pd@H-TpPa with high activity and selectivity
(Figure 4b, entry 2 in Table 1). As the reaction proceeded, the
absorbance of 4-nitrophenol at 400 nm decreased with a con-
comitant increase in the 300 nm peak of 4-aminophenol that
formed after the addition of Pd@H-TpPa. Complete conversion
of 4-nitrophenol into 4-aminophenol within 8 min was observed
by using Pd@H-TpPa. The plot of ln(Ct/C0) versus reaction
time matched the first-order reaction kinetics well (Figure 4c),
and the rate constant (k) was calculated to be 0.41 min−1.
The catalytic activity of Pd@H-TpPa is comparable to that
of previously reported Pd and Au catalysts loaded into dif-
ferent shells for the reduction of 4-nitrophenol by NaBH4.[6a,22]
It is noteworthy that the catalytic activity was affected by the
thickness of the TpPa shell. The reaction rate decreased with
increasing shell thickness (Figures S12–S14, Supporting
Information, entries 3 and 4 in Table 1). Pd@ZIF-8 and Pd@
ZIF-8@Am-TpPa showed lower catalytic activity than Pd@H-
TpPa (Figures S15 and S16, Supporting Information, entries
5 and 6 in Table 1), mainly owing to the physical blockage of the
Pd active sites by ZIF-8. For comparison, core–shell Pd@TpPa
was also synthesized (Figures S17 and S18, Supporting Infor-
mation) and tested in the reduction reaction of 4-nitrophenol
under the same reaction conditions. As shown in Figure 4c
and Figure S19 (Supporting Information), Pd@TpPa afforded
a 75% conversion of 4-nitrophenol after 24 min. The relatively
low catalytic activity of the core–shell Pd@TpPa was mainly due
to the blocked surface of Pd NPs by the COF shell, which sig-
nificantly suppressed the reaction rate.
To confirm that the reduction reaction was indeed catalyzed by
the palladium NPs in the COF cages instead of by leached palla-
dium species, a filtration test was performed. After the reduction
of 4-nitrophenol was run for 2 min, Pd@H-TpPa was removed
from the reaction mixture by filtration. The filtrate was allowed to
react for 30 min, and a negligible amount of further conversion
of 4-nitrophenol was observed (Figure 4d). The concentrations of
palladium in the filtrate were below the detection limit of induc-
tively coupled plasma spectroscopy (ICP), unambiguously proving
the sample heterogeneity as well as the leach-resistant character-
istics of Pd NPs from the COF cages. The catalytic recyclability of
Pd@H-TpPa was further evaluated. Pd@H-TpPa could be readily
recovered by centrifugation after the catalytic reaction. The reco-
vered Pd@H-TpPa could then be dispersed in water by brief soni-
cation and reused for more than four runs without obvious loss
of activity (Figure 4e and Figure S20, Supporting Information).
Scanning electron microscopy (SEM) images and FTIR spectra
show that there was no destruction of the morphological and
structural characteristics of Pd@H-TpPa after being recycled four
times (Figures S21 and S22, Supporting Information), indicating
the stability of the COF shell. Moreover, the Pd NPs were still well
dispersed in the void of the yolk–shell nanocages after recycling
(Figure 4f). No aggregation of the Pd NPs was observed owing to
the interactions between the NPs and the shell of the nanocages.
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Figure 3. a,c) TEM images and b,d) XRD patterns of a,b) Au@H-TpPa and c,d) Ag@H-TpPa.
Thus, the excellent catalytic performance of the Pd@H-TpPa is
mainly ascribed to its well-defined yolk–shell nanostructure, in
which the ligand-free Pd NPs are effectively protected from aggre-
gation and leaching by the COF shell, while the periodic pores
in the COF shell and the void between the COF shell and the
Pd NPs cores enable low mass transfer resistance and improve
the accessibility of the catalytic sites.
this approach can be extended to many other functional COFs.
Accordingly, the functions and properties of these designed
nanocages can be easily tuned by varying the structure of the
COF shell and/or the composition of the core, thus providing
a general strategy for the fabrication of functional materials for
a wide range of applications.
4. Experimental Section
3. Conclusion
Preparation of Pd NPs: Pd NPs were prepared according to literature
methods developed by Xia and co-workers with minor modifications.[23]
In a typical synthesis of Pd cubes/bars, 8.0 mL of an aqueous solution
containing poly(vinyl pyrrolidone) (PVP), 60 mg of l-ascorbic acid,
and 600 mg of KBr were charged into a 20 mL vial. The solution was
preheated under magnetic stirring at 80 °C for 15 min. Next, 3 mL of
another aqueous solution containing 57 mg of K2PdCl4 was added by
using a pipette. The reaction proceeded at 80 °C for 3 h. The products
were collected by centrifugation and washed three times with water to
remove excess PVP, l-ascorbic acid, and KBr. Finally, all of the particles
were dispersed in 10 mL methanol (2.106 mg mL−1).
Preparation of Au NPs: Au nanoparticles were prepared by a sodium
citrate reduction method of HAuCl4.[24] In a typical synthesis of Au NPs,
an aqueous solution of HAuCl4 (0.01%, 150 mL) was brought to a
vigorous boil with rapid stirring in a round-bottom flask (250 mL) fitted
with a reflux condenser. When the solution started to boil, an aqueous
solution of trisodium citrate (1%, 4.5 mL) was added. The mixture
was refluxed with stirring for another 20 min. The resulting deep red
suspension was then removed from heat. After the Au NP solution was
cooled to room temperature, a solution of PVP (0.5 g, Mw = 55 000) in
A facile method to encapsulate various metal nanoparticles
in hollow COF nanocages for catalysis has been developed.
ZIF-8, which was used as a sacrificial template, is etched
spontaneously and simultaneously during the amorphous-to-
crystalline transformation of the polyimine shell. The outer
porous COF shell is permeable, and the MNPs cores can be
accessed by the reactants. Pd@H-TpPa, as a model yolk–shell
structured MNPs@COF nanocage catalyst, shows superior
catalytic activity and stability for the 4-nitrophenol reduction
reaction over that of its core–shell composite counterpart,
which was mainly ascribed to the intrinsic catalytic properties
of Pd NPs with clean surfaces as well as the unique yolk–shell
structures. Importantly, this synthetic strategy is applicable to
a wide variety of nanoparticles, allowing for engineering of
the active MNP cores for the targeted heterogeneous catalytic
reaction through a rational catalyst design. We also believe that
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Figure 4. a,b) Time-dependent UV–vis absorption spectra for the catalytic reduction of 4-nitrophenol without and with Pd@H-TpPa; c) catalytic conver-
sion of 4-nitrophenol using Pd@H-TpPa and Pd@TpPa, inset: ln(Ct/C0) versus time; d) filtration test and e) recyclability of Pd@H-TpPa for the reduc-
tion of nitrobenzene; f) TEM image of the recovered Pd@H-TpPa after the fourth run reaction. Reaction conditions: Pd composites (5 mL, 2 mg mL−1),
4-nitrophenol (8 mL, 0.18 × 10−3 m), and NaBH4 (6 mL, 2 mg mL−1) in water at room temperature. The conversion of 4-nitrophenol to 4-aminophenol
was monitored by UV–vis spectra at short intervals in the range 250–550 nm.
water (20 mL) was added dropwise to the Au NP solution with stirring,
and the mixture was further stirred at room temperature for 24 h. The
PVP-stabilized Au NPs were collected by centrifugation at 14 000 rpm
for 30 min, washed with methanol three times, and finally dispersed in
methanol.
Preparation of Ag NPs: Ag NPs were prepared according to literature
methods.[25] Anhydrous ethylene glycol (5 mL) was heated at 160 °C for 1 h.
An ethylene glycol solution of AgNO3 (0.25 m, 3 mL) and an ethylene glycol
solution of PVP (0.375 m, Mw = 55 000, 3 mL) were simultaneously added
to the heated ethylene glycol using a two-channel syringe pump at a rate of
0.375 mL min−1. The reaction mixture was heated at 160 °C for 45 min. The
mixture was then diluted with water and filtered through a 1 µm membrane.
Silver nanocubes were collected by centrifugation, washed three times with
methanol, and finally dispersed in methanol (0.2 mg mL−1).
Table 1. Summary of reduction of 4-nitrophenol.
Entry
Catalysta)
Time
[min]
Yield
[%]
1b)
2c)
3
–
12
8
0
Pd@H-TpPa
96%
>99%
67%
92%
42%
Pd@H-TpPa with 15 nm shell thickness
Pd@H-TpPa with 75 nm shell thickness
Pd@ZIF-8
4
4
32
8
5
6
Pd@Am-TpPa
32
a)Catalytic performances of Pd composites (5 mL, 2 mg mL−1) in 4-nitrophenol
(8 mL, 0.18 × 10−3 m) reduction in water by NaBH4 (6 mL, 2 mg mL−1) at room
temperature. The conversion of 4-NPh to 4-aminophenol was monitored by
UV–vis spectra in the range 250–550 nm; b)Without Pd catalyst; c)The shell thick-
ness is 26 nm.
Preparation of Pd@ZIF-8: A 0.5 mL Pd NP solution at the desired
concentration, 5 mL solution of 2-methylimidazole (25 × 10−3 m), and 5 mL
solution of Zn(NO3)2 ⋅ 6H2O (25 × 10−3 m) were mixed and allowed to
react at room temperature for 24 h without stirring. The product was
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collected by centrifugation, washed several times with methanol, and
vacuum-dried overnight. All solutions used methanol as the solvent.
Preparation of Pd@ZIF-8@Am-TpPa: A tetrahydrofuran solution (50 mL)
containing Pd@ZIF-8 (50 mg) and Pa (15 mg, 0.14 mmol) was stirred
for 5 min. Next, 5 mL of another tetrahydrofuran solution containing
Tp (20 mg, 0.1 mmol) was added slowly. The reaction proceeded under
magnetic stirring for 24 h. The products were collected by filtering, washed
three times with tetrahydrofuran, and vacuum-dried overnight.
Preparation of Pd@H-TpPa: A 10 mL glass vial was charged with Pd@
ZIF-8@Am-TpPa (50 mg), 1,4-dioxane (2 mL), and aqueous acetic acid
(3 m, 350 µL). The vial was then sealed and left undisturbed for 3 d at
room temperature. The yellow precipitate was collected by centrifugation
and washed with tetrahydrofuran, water, and acetone, separately. The
collected powder was dried at 60 °C under vacuum for 12 h to give a
yellow powder with an 80% isolated yield. The Pd content in Pd@H-
TpPa was 5 mmol g−1, as determined by ICP.
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containing Pd NPs (20 mL) and Pa (15 mg, 0.14 mmol) was stirred for
5 min. Next, 5 mL of another tetrahydrofuran solution containing Tp
(20 mg, 0.1 mmol) was slowly added. The reaction proceeded under
magnetic stirring for 24 h. The products were collected by filtering,
washed three times with tetrahydrofuran, and vacuum-dried overnight.
Then, a 10 mL glass vial was charged with the as-synthesized products
(50 mg), 1,4-dioxane (2 mL), and aqueous acetic acid (3 m, 350 µL). The
vial was then sealed and left undisturbed for 3 d at room temperature.
The collected powder was dried at 60 °C under vacuum for 12 h.
Reduction of 4-Nitrophenol by Pd@H-TpPa: Typically, the catalytic
process proceeded under ambient conditions. First, 6 mL of NaBH4
(2 mg mL−1) and 8 mL of 4-nitrophenol (0.18 × 10−3 m) in water were
mixed in a flask. Then, a mixture of 5 mL of catalyst (2 mg mL−1) was
added to the solution. After adding the as-prepared catalyst, the color
of the 4-nitrophenol solution gradually faded from bright yellow to
colorless as the reaction proceeded. The conversion of 4-nitrophenol to
4-aminophenol was monitored by recording the UV–vis spectra at short
intervals in the range of 250–550 nm. On the basis of the change in the
spectral intensity at λ = 400 nm as a function of time, the rate constants
for the catalytic hydrogenation of 4-nitrophenol were determined.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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Acknowledgements
The authors thank the National Natural Science Foundation of China
(Nos. 51672046, 51672047, 51472050, and 21403238), the Open Project
Program of the State Key Laboratory of Photocatalysis on Energy and
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Conflict of Interest
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Keywords
covalent organic frameworks, metal nanoparticles, nanocages,
nanoreactors, yolk–shell
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Received: October 23, 2018
Revised: November 22, 2018
Published online:
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2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim