MOFs-Templated Co@Pd Core-Shell NPs Embedded in N-Doped Carbon Matrix with Superior Hydrogenation Activities

Article abstract of DOI:10.1021/acscatal.5b00998

Although MOFs supporting noble metal nanoparticles (NPs) have been widely used in heterogeneous catalysis, they are still limited in catalytic efficiency on a per-noble-metal-atom basis. Here we developed a MOFs-templated strategy to non-noble metal @noble metal core-shell nanocatalysts, which could far surpass the traditional MOFs supporting noble NPs in catalytic properties, by using MOFs-derived metal NPs as sacrificial templates to reduce noble metal ions via galvanic replacement reaction. As a model system, Co@Pd core-shell NPs embedded in the N-doped carbon matrix (Co@Pd/NC) were synthesized with an average size of ca. 9.4 nm and a ultrathin Pd shell by using ZIF-67 and Pd(NO₃)₂ as the precursor and Pd source, respectively. The highly exposed Pd atom on Co nanoparticles made it an attractive catalyst with high efficiency. When being used in the hydrogenation of nitrobenzene, the Co@Pd/NC exhibited an unprecedented high activity over Pd-based catalysts, yielding 98% conversion after 45 min reaction, which was far more active than the pristine ZIF-67 and MIL-101 supporting Pd NPs (with 40% and 3% conversion after 90 min reaction, respectively) under identical conditions, suggesting its overwhelmingly better performance than MOFs supporting noble NPs. We anticipated that this strategy would form the basis for developing a new class of MOF-templated core-shell nanocatalyst with potential applications in numerous catalytic reactions.

Full text of DOI:10.1021/acscatal.5b00998

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ACS Catalysis
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MOFs-Templated Co@Pd Core-Shell NPs Embedded in N-
doped Carbon Matrix with Superior Hydrogenation Activities
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Kui Shen,† Liang Chen,‡ Jilan Long,† Wei Zhong† and Yingwei Li†*
†
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Peo-
ple’s Republic of China
‡Department of Chemistry and Collaborative Innovation Center of Chemistry for Energy Materials, Fudan Universi-
ty, 220 Handan Road, Shanghai 200433, People’s Republic of China
ABSTRACT: Although MOFs supporting noble metal nanoparticles (NPs) have been widely used in heterogeneous cataly-
sis, they are still limited in catalytic efficiency on a per noble metal atom basis. Here, we developed a MOFs-templated
strategy to non-noble metal @noble metal core-shell nanocatalysts, which could far surpass the traditional MOFs sup-
porting noble NPs in catalytic properties, by using MOFs-derived metal NPs as sacrificial templates to reduce noble metal
ions via galvanic replacement reaction. As a model system, Co@Pd core-shell NPs embedded in the N-doped carbon ma-
trix (Co@Pd/NC) were synthesized with an average size of ca. 9.4 nm and a ultrathin Pd shell by using ZIF-67 and
Pd(NO ) as the precursor and Pd source, respectively. The highly exposed Pd atom on Co nanoparticles made it an at-
3
2
tractive catalyst with high efficiency. When being used in the hydrogenation of nitrobenzene, the Co@Pd/NC exhibited
an unprecedented high activity over Pd-based catalysts, yielding 98% conversion after 45 min reaction, which was far
more active than the pristine ZIF-67 and MIL-101 supporting Pd NPs (with 40% and 3% conversion after 90 min reaction,
respectively) under identical conditions, suggesting its overwhelmingly better performance than MOFs supporting noble
NPs. We anticipated that this strategy would form the basis for developing a new class of MOF-templated core-shell
nanocatalyst with potential applications in numerous catalytic reactions.
KEYWORDS: Core-shell structure, heterogeneous catalysis, metal-organic frameworks, nanoparticles, noble metal.
1
2
and poor electrical conductivity, which hinder their ap-
plications for catalysis; (2) The noble NPs supported on
MOFs by the existing methods are mostly several na-
nometers in diameter, resulting in the lack of sufficient
active centers and the optimal microstructure. Apparent-
ly, alternative concepts and strategies have to be devised
to break the above two bottleneck and develop novel
supported noble metal catalysts with high efficiencies.
INTRODUCTION
Metal-organic Frameworks (MOFs) built from metal
ions and polyfunctional organic ligands are attractive and
significant materials for many industrial applications due
to their excellent chemical and physical properties such as
extra high specific surface area, well defined pore struc-
3
-7
1
ture and low density. Particularly, MOFs provide a tune-
able coordination space and chemical tailoring of the in-
ner surface of the channels and cavities, which make
them a promising new class of catalyst supports for host-
More recently, MOFs were proved to be ideal sacrificial
templates for fabricating porous carbon-based nano-
materials via thermal decomposition under controlled
ing nanoparticles, especially for noble metal nanoparticles
8b, 13
atmospheres.
2
In particular, by calcining the MOFs
(
MNPs) in heterogeneous catalysis. Over the past dec-
containing the metal centers with a reduction potential of
0.27 volts or higher in inert atmosphere, these metal ions
of MOFs could be transformed into metal NPs, which are
ades, research efforts have been mostly aimed at prepar-
ing new MOFs structures and developing new approaches
for loading MNPs more effectively inside the porous ma-
14
embedded in a ligand-derived porous carbon matrix.
2
trices of MOFs. A few examples are Pd, Au, Pt, Ru, Ir, Au-
Inspired by these advances, herein, we propose a facile
and efficient approach to fabricate non-noble met-
al@noble metal core-shell nanocatalysts by using MOFs-
derived metal NPs as sacrificial templates to reduce noble
metal ions via galvanic replacement reaction. Co@Pd
core-shell nanoparticles embedded in N-doped carbon
matrix (denoted as Co@Pd/CN) were successfully pre-
pared as a model to verify this strategy by using ZIF-67
and Pd(NO ) as the precursor and Pd source, respectively.
Pd, Ni-Pd nanoparticles stabilized on different MOF
structures by employing various techniques such as
3
4
chemical vapor deposition (CVD), impregnation, solid
5
6
grinding, colloidal deposition method and double sol-
7
vents approach. However, in spite of these promising
approaches, the MOFs supporting noble NPs are still lim-
ited in catalytic performance because of the following two
reasons: (1) Most of MOFs are known to had their intrin-
sic weaker material properties such as low thermal stabil-
3
2
8
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10
11
11
The highly exposed Pd atom on Co nanoparticles made it
excellent as a highly active, extremely stable nanocatalyst
ity, extreme sensitivity to air, water, acids, and bases,
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toward the nitrobenzene hydrogenation reaction. The
Co@Pd/NC showed 98% conversion of nitrobenzene after
5 min reaction, which was far more active than the pris-
tine ZIF-67 and MIL-101 supporting Pd NPs (with 40% and
% conversion after 90 min reaction, respectively) under
bly filtered off using two glass filters with pore sizes be-
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tween 40 and 100 μm to remove the unreacted crystals of
terephthalic acid, and further soaked in ethanol (95%
EtOH with 5% water) at 80 °C for 24 h. Finally, the result-
ing MIL-101 was dried overnight at 150 °C under vacuum
for further use.
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identical conditions.
Synthesis of Pd/MIL-101, Pd/ZIF-67, Pd/C, Pd/NC
and Pd-Co/MIL-101. All of Pd/MIL-101, Pd/ZIF-67 and
Pd/C were prepared via a conventional impregnation
method by using MIL-101, ZIF-67 and activated charcoal
as the supports. Typically, 0.6 g of support (MIL-101/ZIF-
EXPERIMENTAL SECTION
Chemicals. Methylimidazole (MeIM, 98%), Cobalt ni-
trate (Co(NO ) .6H O, >99%) benzenedicarboxylate
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(
H bdc, >99%) and Chromium(III) nitrate nonahydrate
2
were purchased from Aladdin Industrial Corporation.
Palladium nitrate dihydrate was purchased from Shanghai
yuanye Bio-Technology Co., Ltd. The activated charcoal
was purchased from Sigma-Aldrich.
6
7/activated charcoal) was dispersed in 20 mL of acetone
and was stirred for 20 min at room temperature. 5 mL
2+
Pd acetone solution containing 0.03 g of Pd(NO ) .2H O
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2
2
was added dropwise to the above solution under magnetic
stirring for about 10 min. The flask containing the slurry
was subjected to ultrasonication for 20 min and was then
magnetically agitated at room temperature for 24 h. The
impregnated samples were washed with acetone till the
filtrate became colorless and then was slowly dried under
air at room temperature for 24 h. The as-synthesized
Synthesis of ZIF-67. The preparation of ZIF-67 polyhe-
drons (Co(MeIM) , MeIM=2-methylimidazole) was based
2
1
5
on a reported method with some modifications. In a typ-
ical synthesis, 0.45 g of cobalt nitrate hexahydrate was
dissolved in 3 mL of deionized water. Then another solu-
tion was prepared by dissolving 5.5
g
of 2-
samples were further dried at 150 °C for 8 h, followed by
methylimidazole in 20 mL of deionized water. The former
solution was poured into the later one under magnetic
stirring, resulting in a mixture with molar composition of
o
treating in a stream of H at 200 C for 2 h to obtain
2
Pd/MIL-101, Pd/ZIF-67, and Pd/C. In order to study the
role of N in the activity of Co@Pd/NC, we also prepared
Pd/NC by the above impregnation method with NC as the
support, which was obtained by etching away Co NPs in
Co/NC using aqua regia. Based on Atomic Absorption
Spectroscopy (AAS) analysis, the Pd loadings in the
Pd/MIL-101, Pd/ZIF-67, Pd/C and Pd/NC were 1.02, 0.87,
2+
1 Co : 58 MeIM: 1100 H O. After keeping stirring for 6 h at
2
room temperature, the resulting purple precipitates were
washed thoroughly with water, and finally dried under
vacuum at 80 °C for 24 h.
Synthesis of Co/NC materials by thermolysis of ZIF-
67. The typical procedure for the preparation of the
1
.21 and o.71 wt%, respectively. Besides, Pd-Co/MIL-101
Co/NC materials was described as follow: 2 g of ZIF-67
was also prepared by a colloidal deposition method. Brief-
ly, 0.059 g of PVP (MW=24000) was added to an aqueous
solution (20 mL) containing 0.005 g Pd(NO ) .2H O and
o
o
was pyrolyzed at 600 C with a heating rate of 1 C/min
from room temperature under continuous Ar flow. After
reaching the target temperature, the material was held at
this temperature for 8 hours and then cooled to room
temperature.
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.0099 g Co(NO ) .6H O. The mixture was stirred for 0.5
3 2 2
h at 0 °C. Then, a freshly prepared aqueous solution of
NaBH4 (0.02 g) was rapidly added to the above mixture
under vigorous stirring. After sol formation in a few
minutes, 0.2 g activated MIL-101 was immediately added,
and the solution was further stirred for 10 h. After wash-
ing thoroughly with deionized water, the obtained solid
was further dried at 80 °C for 5 h under vacuum to obtain
the final Pd-Co/MIL-101 (the Pd and Co loadings in the
Pd-Co/MIL-101 were 0.87, and o.61wt%, respectively).
Synthesis of Co@Pd/NC based on galvanic replace-
ment reaction. In a standard procedure, 0.4 g of as-
prepared Co/NC was initially dispersed in 20 ml of ace-
tone to form a uniform suspension via ultrasonication for
5
min. Then, 5 ml of acetone solution containing required
amount of Pd(NO ) was added dropwise under magnetic
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stirring at room temperature. The resultant mixture was
reacted for another 12 h. All the synthetic processes were
conducted under an Ar atmosphere as the Co NPs were
easily oxidized in air. The Co@Pd/NC formed was recov-
ered by an external magnetic field, washed several times
Characterization. Powder X-ray diffraction patterns of
the samples were obtained on a Rigaku diffractometer
(D/MAX-IIIA, 3 kW) using Cu Kα radiation (40 kV, 30 mA,
λ = 0.1543 nm). BET surface area and pore size measure-
ments were performed with N2 adsorption/desorption
isotherms at 77 K on a Micromeritics ASAP 2020 instru-
ment. Before measurements, the samples were degassed
at 100 °C for 12 h. TGA-DSC of ZIF-67 was performed on a
NETZSCH STA449C under argon atmosphere. X-Ray pho-
toelectron spectroscopy (XPS) measurements were per-
formed on a Kratos Axis Ultra DLD system with a base
o
with acetone and dried under vacuum at 50 C.
Synthesis of MIL-101. MIL-101 was synthesized by the
1f
hydrothermal method according to a previous literature.
Cr(NO ) .9H O (99%) (2.007 g, 5.0 mmol), HF (48 wt.%,
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5
.0 mmol), terephthalic acid (98%, 0.823 g, 5.0 mmol),
and 24 mL of deionized water were mixed completely and
heated at 220 °C for 8 h. And then, the mixture was
cooled slowly to room temperature. The resulting green
−
9
pressure of 10 Torr. The Co and Pd contents in the sam-
ples were measured quantitatively by atomic absorption
spectroscopy (AAS) on a HITACHI Z-2300 instrument.
powder
of
MIL-101
with
formula
Cr F(H O) O[(O C)C H (CO )] .nH O (n ≤ 25) was dou-
3
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2
6
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Elemental analysis was performed on a Elementar Vario
EL III equipment. The size and morphology of materials
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were studied by high-resolution scanning electron mi-
croscopy (HR-SEM, S-3700N of HITACHI) equipped with
an energy dispersive X-ray spectroscopy (EDX), and high-
resolution transmission electron microscopy (HR-TEM,
C/M300 of Philips). The elemental mappings were per-
formed on a scanning transmission electron microscope
(
STEM) unit with high-angle annular dark-field (HAADF)
detector (HITACHI S-5500) operating at 200 kV. The
samples for TEM test were prepared by dissolving the
samples in ethanol and then ultrasound for 30 minutes.
After that, a very small amount of suspensions were taken
out using microsyringe and then dropped on a copper
online. Raman spectra of the carbonized samples were
obtained at room temperature with a LabRAM Aramis
Raman system with a laser of 532 nm.
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Hydrogenation of nitrobenzene. Typically, calculated
amount of Co@Pd/NC (0.2 mol%) in ethyl acetate (2 ml)
was placed in a 25 mL Schlenk tube and nitrobenzene
Figure 1. Schematic illustration of the process used for the
synthesis of Co@Pd/NC.
(
0.062 g, 0.5 mmol) was added to the mixture under H (1
2
resulting nitrogen−carbon composite played an important
role in confining the Co species, leading to high disper-
sion of the Co NPs throughout carbon matrix with less
aggregation. Since the standard reduction potential of the
atm) with a magnetic stirrer to initiate the reaction. After
completion of the reaction, the catalyst was removed
from the solution by magnetic field and the liquid phase
was subsequently analyzed by GC/MS (Shimadzu GCMS-
QP5050A equipped with a 0.25 mm × 30 m DB-WAX ca-
pillary column). In addition to nitrobenzene, an array of
nitro-substrates was also subjected to the Co@Pd/NC
using the same Pd/substrate molar ratio. The recyclability
of the Co@Pd/NC catalyst was investigated for the hydro-
genation of nitrobenzene under an appropriate reaction
condition (0.67 mmol nitrobenzene in 2ml EtOAc, mag-
2+
Co/Co pair (-0.28 V vs. the standard hydrogen electrode,
2+
SHE) was much lower than that of the Pd /Pd pair (0.8 V
2
+
0
vs. SHE), Pd were reduced to Pd (by the galvanic dis-
0
2+
2+
0
placement reaction: Co + Pd → Co + Pd ) and depos-
ited on the surface of Co NPs, when Co/NC and Pd(NO3)
2
were mixed in an aqueous medium, forming novel Co@Pd
core-shell nanomaterial. The Co and Pd contents in the
Co@Pd/NC materials were 30.7 wt % and 1.1 wt%, respec-
tively (Table S2). For comparison, Pd NPs supported ZIF-
netically stirring, H balloon, room temperature, catalysts:
2
0
.15 mol% Pd, 45 min of reaction time) by using the re-
6
7, MIL-101 and activated charcoal (denoted as Pd/ZIF-67,
covered catalysts. Each time, the catalyst was isolated
from the solution by magnetic field after reaction, washed
several times with ethyl acetate, dried under vacuum to
remove the residual solvent, and then reused as the cata-
lyst in the next run.
Pd/MIL-101, and Pd/C, respectively) were also prepared by
conventional impregnation method.
The key idea of our method for synthesis of the
Co@Pd/NC was to exploit the efficient growth of Pd shell
on the core Co NPs through the replacement reaction.
Obviously, smaller Co nanoparticles with higher active
surface should be beneficial for the deposition of ultrathin
Pd layer and thereby for enhanced catalytic activity. Thus,
we initially studied the effect of pyrolysis temperature on
the properties of resultant products. According to the
TGA-DSC curves (Figure S2), the ZIF-67 precursor was
RESULTS AND DISCUSSION
Figure 1 schematically illustrated the procedure used
herein to prepare Co@Pd/CN. A well-studied zeolitic im-
idazolate framework material, ZIF-67, was first prepared
from cobalt nitrate and 2-methylimidazole according to a
1
4d, 15
facile room-temperature method.
XRD patterns of as-
o
highly stable up to 570 C where they began to decompose.
synthesized ZIF-67 (Figure 2d and Figure S1) matched
exactly with the simulated and also the published XRD
So, four different calcination temperatures (600, 700, 800,
and 900 °C) were applied with a heating rate of 1 °C/min
from room temperature. The TEM images and corre-
sponding size distribution of the resultant products (Fig-
ure 3) revealed that the average diameter of the Co NPs
gradually increased from ca. 9 to ca. 27 nm with increas-
1
6-17
patterns,
confirming the product as phase-pure ZIF-67
with good crystallinity. SEM (Figure 2a and S5a) images
clearly revealed the formation of polyhedral ZIF-67 with
smooth surface. Co NPs embedded in N-doped carbon
were then synthesized by carbonizing ZIF-67 in an inert
gas. During the pyrolysis process, the ligands of ZIF-67 (2-
methylimidazole) were subjected to carbonization, while
o
ing pyrolysis temperature from 600 to 900 C. This struc-
tural change was also reflected in the XRD patterns. As
shown in Figure S3, all XRD patterns of the calcined com-
posites showed three diffraction peaks at around 44.2, 51.6,
2
+
14a, 14d
the Co species were reduced to metallic Co.
The
o
and 76.0 , respectively, which were characteristics of
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tion temperature to be 600 °C and the resulting sample
was denoted as Co/NC.
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As observed from Figure 2b and 2c, both the Co/NC
and Co@Pd/NC retained the similar size (∼150 nm) and
shape (concave cubes) as the ZIF-67 precursors. However,
the surface of the particles became fairly rougher than the
ZIF-67 precursor, indicating the complete decomposition
and carbonization of the zeolitic imidazolate framework.
Compared to strong peaks that originated from ZIF-67 in
XRD patterns (Figure 2d), characteristic peaks associated
with metallic Pd within the Co@Pd/NC were too weak to
be observed clearly, presumably because of their low con-
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22
centrations and/or small sizes. However, their existence
was confirmed by XPS (Figure 2e and S6) and AAS anal-
yses (Table S2). In the Pd 3d XPS spectra of Co@Pd/NC
illustrated in Figure 2e, two obvious peaks of Pd 3d5/2 and
Pd 3d3/2 appeared at binding energies of 335.76 and 341.06
eV, respectively, which were close to, but lower by ∼0.3
0
eV than that of monometallic Pd on activated charcoal
(
Pd/C, 336.06 and 341.36 eV). This implied that the Pd was
23
existed in the metallic form in the Co@Pd/NC. The peak
shift of Pd to lower values might be attributable to the
0
ultrafine size of Pd shell and the additional electron do-
24
nating effect from inner Co atom to shell Pd atom. In-
terestingly, the surface atomic ratio of Pd to Co was de-
termined to be 0.21 from their peak areas after correction
of sensitivity factors (Figure 2e, S6 and S7), which was
much higher than the Pd/Co ratio determined in the bulk
by AAS measurement (0.018) and TEM-EDX analysis
Figure 2. Typical SEM images of (a) ZIF 67, (b) Co/NC and
c) Co@Pd/NC. (d) XRD patterns of ZIF-67, Co/NC and
Co@Pd/NC. (e) Pd 3d XPS spectra of Co@Pd/NC and
Pd/C. (f) N 1s XPS spectra of Co@Pd/NC and Co/NC.
(
1
4d, 18
metallic Co (JCPDS No. 15-0806).
Note that the higher
(
0.024) (Table S2 and Figure 4n), indicating that Co cores
the calcination temperature, the narrower the peak width
and the stronger the peak intensity. These suggested that
the Co NPs in the products turned bigger in size and the
crystallinity became better as the calcination temperature
increased. According to the Scherrer equation (using the
had been covered by Pd shell. The N 1s XPS spectra of
Co/NC (Figure 2f) further shown the presence of two
types of nitrogen species, which could be attributed to
pyridinic (399.1 eV) and graphitic (401.0 eV) nitrogen at-
25
oms, indicating that the N atoms were doped successful-
1
9
44.2° (111) peak), the average size of the Co NPs obtained
at 600 °C was estimated to be 8.1 nm, much smaller than
that (24.6 nm) of the sample obtained at 900 °C, which
were nearly matched with the observed values from TEM.
Raman spectroscopy had also been performed to deter-
mine the structural features of the carbonized samples
(Figure S4). As expected, two predominant peaks at 1339
ly into the Co/NC. However, the binding energy of pyri-
dinic N shifted to higher value than those of metal-free N-
25
doped graphene, probably due to the electron redistri-
bution after ionization and strong coordination interac-
26
tion between Co and N atoms (Figure S8). As expected,
coverage of Co cores by Pd shells gently weakened the
coordination interaction between Co and N atoms, as
declared by the peak shift of pyridinic N for Co@Pd/NC
to lower values compared with Co/NC. The further quan-
titative analysis of the XPS results revealed that the N
contents in Co/NC and Co@Pd/NC were as high as 7.9
and 7.4 wt%, respectively. These findings illustrated the
absence of oxygen atoms in ZIF-67 precursor could retain
effectively the nitrogen content in the final material.
-
1
-1
cm and 1581 cm were observed for all samples, corre-
sponding to the D and G bands, respectively. The relative
ratios of the D band to the G band illustrated the degree
20
of graphitization. The I /I of Co/NC (namely the sam-
D
G
o
ple prepared by carbonizing ZIF-67 at 600 C for 8 h) was
around 1.57, which suggested the graphitic character of
carbon species in Co/NC was not well developed, and the
obtained carbon structures contain both graphene layers
and disordered carbons. Furthermore, increasing the car-
bonizing temperature and the duration of calcination
both slightly increased the degree of graphitization of the
obtained porous carbon. In addition, the low calcination
temperature were obviously in favour of retaining nitro-
gen content of the final material (Table S1), which were
widely regarded as an effective way to enhance the per-
More structural informations of Co@Pd/NC were further
revealed by TEM characterization. Ultrasonic treatment
(30 min) of the sample for TEM tests was unable to break
the Co@Pd/NC particles, indicating their good mechani-
cal stability (Figure 4a). As shown in Figure 4b, Co@Pd
NPs were highly dispersed and embedded in a ligand-
derived porous carbon matrix. The average size of Co@Pd
NPs was determined to be 9.4 nm (Figure 4b), similar to
21
formance of carbon materials. Thus we set the calcina-
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Figure 3. TEM images and corresponding size distribution of Co NPs synthesized by carbonizing the ZIF-67 precursor at
different temperature: (a) 600 C, (b) 700 C, (c) 800 C and (d) 900 C.
o
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o
o
Figure 4. (a) and (b) TEM images of the Co@Pd/NC. The inset showed the corresponding size distribution of Co@Pd NPs.
(
c) HRTEM image of a Co@Pd nanoparticle. (d-g) Low-magnification EDX spectra of Co, Pd, C and N for a Co@Pd/NC
particle. (h) HAADF-STEM image of an individual Co@Pd NPs and (i-k) the corresponding STEM-EDX elemental map-
ping images of Co and Pd. (m) Elemental line-scanning profiles along the direction marked by a white line in (l). (n) EDX
spectrum of the Co@Pd/NC.
that of starting Co nanoparticles, since shell layer for-
mation and core metal consumption occurred simultane-
ously. Figure 4c highlighted the typical high resolution
TEM (HRTEM) image of a Co@Pd particle, which was
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embedded in layered graphitic carbon structures with a
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typical distance value of graphite (0.336 nm). It was
worth noting that only one type of crystal lattice was ob-
served on the Co@Pd nanoparticle. These clearly-resolved
interplanar fringes were measured to be 0.223 nm, which
should be ascribed to the crystal lattice of the (111) planes
1
8
of face-centered cubic (fcc) Co. This meant the host Co
matrix was well retained. The absence of Pd interplanar
fringes should be due to the ultrafine size of Pd shell.
Low-magnification EDX images (Figure 4d-4g) confirmed
the homogeneous three-dimensional distribution of the
Pd throughout the interior of the Co@Pd/NC particles
and high-magnification EDX images (Figure 4h-4k) clear-
ly revealed the coexistence of Co and Pd in an individual
nanoparticle, which was surrounded by N-doped carbon.
The presence of uniform Pd and Co in this nanoparticle
was consistent with either CoPd alloys or the formation of
the proposed Co@Pd core-shell particles. However, the
EDX line scan (Figure 4m) taken along the line marked in
Figure 4l showed a Co-rich core and an ultrathin Pd-rich
shell, indicating the successful preparation of Co@Pd
core-shell nanomaterial. From the EDX spectra in Figure
4n, Pd, Co, C and N signals could be observed, reconfirm-
ing the existence of these four elements. The Pd/Co atom-
ic ratio was determined to be 0.024, roughly consistent
with the measured ratio of 0.018 by AAS measurement.
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Figure 5. (a) Nitrogen adsorption/desorption isotherms at
77 K and Horvath−Kawazoe pore-size distribution curves
(inset) of the ZIF-67, Co/NC and Co@Pd/NC. (b) The Pd
and Co actual content of the final products as a function
Pd addition content.
The polymerization and carbonization of imidazolate
framework produced substantial meso/macropores, lead-
ing to a different pore structure from the ZIF-67 precursor.
As shown in Figure 5a, the curves for ZIF-67 were a bit
2
–1
1
6-17, 28
decreased from 239.7 to 59.7 m g . These results indicated
the significant decrease of BET for Co@Pd/NC was caused
by the destructive effect of acetone on the micropores
rather than the Pd deposition, possibly due to capillary-
force-driven channel collapse during removal of acetone
solvent. It is well known that galvanic replacement reac-
tion has emerged as a powerful synthetic approach for
similar to the I-type isotherms,
suggesting its domi-
nating microporous characteristic. However, isotherms of
the Co/NC exhibited a typical adsorption curve of type I
plus IV with an apparent enhanced uptake and an obvious
1
4d, 28
hysteresis loop in the P/Po range of 0.8−1,
indicating
its remarkable micro-/meso-/macroporous structure (in-
set in Figure 5a and Figure S9). Interestingly, with the
introduction of Pd in Co/NC, a huge drop in surface area
was observed mainly from a decrease in micropore sur-
face area (Table S2). As a result, the Co@Pd/NC showed a
30
converting solid metal nanostructures into hollow ones.
However, a control experiment with different Pd invento-
ry indicated the galvanic replacement reaction could
reach the thermodynamic equilibrium in our work with a
maximal Pd loading of ca. 1.1 wt% (Figure 5b), indicating
the surface coating with N-doped carbon shells could ef-
fectively facilitate the formation of ultrathin Pd shell and
thus minimize the use of Pd. In addition, with increasing
the Pd dosage from 0.5 to 2 wt%, the Pd actual content of
the final product increased from 0.3 to 1.1 wt% and the Co
actual content decreased from 31.5 to 30.3 wt%, signifying
the gradual galvanic replacement of Co by Pd.
3
meso/macropore volume of 0.34 cm /g, which was similar
3
to that of Co/NC (0.37 m /g) and 3.9-fold higher than that
3
of the ZIF-67 precursor (0.09 m /g). Since the larger pores
29
remarkably facilitated physical transport, the accessible
meso-/macroporous structure of Co@Pd/NC could effec-
tively enhance the mass and heat transfer of guest mole-
cules and also the utilization of catalyst. In order to un-
cover the reason why the surface area dropped so signifi-
cantly after Pd deposition, we conducted another com-
parative experiment where 0.4 g of as-prepared Co/NC
was initially dispersed in 20 ml of acetone via ultrasoni-
cation and then agitated under magnetic stirring at room
temperature for 12 h. After recovering, washing and dry-
ing, BET surface area and pore size measurements were
measured over the obtained solid (denoted as Co/NC-ace)
The catalytic activities of the Co@Pd/NC were tested
and compared with the pristine ZIF-67 supporting Pd NPs
(
Pd/ZIF-67, Figure S11), the most-used MIL-101 supporting
Pd NPs (Pd/MIL-101, Figure S12), and Pd-Co NPs (Pd-
Co/MIL-101, Figure S13), the activated charcoal supporting
Pd NPs (Pd/C, Figure S14) and the Co-free NC supporting
Pd NPs (Pd/NC, Figure S15), using the hydrogenation of
nitrobenzene as a model reaction. Reactions were per-
formed at room temperature and atmospheric pressure of
by N adsorption/desorption isotherms. As observed from
2
Figure S10, the N adsorption/desorption isotherms of the
2
Co/NC-ace was very similar to that of Co@Pd/NC. Ac-
cordingly, the BET surface area of Co/NC-ace also
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Figure 6. (a) Nitrobenzene conversions over various catalysts as a function of reaction time. (b) Effect of Pd content on
the catalytic performance of the Co@Pd/NC-X (X was the Pd content). (c) Magnetic separation of the Co@Pd/NC. (d)
Recyclability of the Co@Pd/NC in the hydrogenation of nitrobenzene.
H under base-free conditions. As shown in Figure 6a, the
The recyclability of the Co@Pd/NC was further investi-
gated under an appropriate reaction condition (0.67
mmol nitrobenzene in 2mL EtOAc, magnetically stirring,
2
Pd/C and Pd/ZIF-67 were almost inactive for this reaction.
After 90 min of reaction, the conversion of nitrobenzene
over Pd/C and Pd/ZIF-67 were only 9% and 3%, respec-
tively. The Pd/MIL-101 as a conventional hydrogenation
catalyst showed moderate activity, giving 40% conversion
of nitrobenzene after 90 min of reaction. In sharp con-
trast, the Co@Pd/NC exhibited extremely high activity
with a 98% conversion of nitrobenzene within only 45
min of reaction. In fact, our Co@Pd/NC catalyst was also
the most active for hydrogenation of nitrobenzene among
a number of supported Pd catalysts (Table S3). The high
activity of Co@Pd/NC must have been originated from
H balloon, room temperature, catalysts: 0.15 mol% Pd, 45
2
min of reaction time). After the reaction, the Co@Pd/NC
was easily separated from the reaction solution by the
magnet (Figure 6c), implying its good magnetic proper-
ties. As shown in Figure 6d, the Co@Pd/NC could be re-
generated and reused at least seven times in the subse-
quent reaction without significant decreases in catalytic
activity and selectivity. In addition, after removing the
solids from the reaction mixture after 15 min of reaction
time and stirring the reaction solution for additional 30
minutes, no further conversion of nitrobenzene was ob-
served, which confirmed that the reaction catalyzed by
Co@Pd/NC was truly heterogeneous. This finding was in
good agreement with AAS experiments where no Pd trac-
es were detected in the reaction solution. Further investi-
gation of the substrate scope was performed under the
same reaction conditions. As summarized in Table 1, our
Co@Pd/NC showed good tolerance to a broad scope of
substituted nitroarenes containing -OH, -CHO, -CN, -CO,
-COOC, -CON and -X functional groups. For all sub-
strates, the selectivity to functionalized anilines was ≥
98% at the nitroarene conversion level of ≥93%. Besides,
the slowly-dissolving nitroarenes containing Br or Cl
groups (Table 1, entries 13 and 14) needed much more
time to complete than the F-substituent nitroarene,
which has a high solubility in EtOAc (entry 12).
3
1
the highly exposed Pd atom on Co cores, as our prelimi-
nary test showed that the Co/NC itself was essentially
inactive for this reaction under the investigated condi-
tions. The role of N in the Co@Pd/NC’s activity was eval-
uated by a referential catalyst prepared by first etching
away the Co NPs in Co/NC and then loading Pd NPs over
the obtained N-doped porous carbon (namely Pd/NC).
Interesting, the Pd/NC showed slightly higher hydrogena-
tion activity than the Pd/C, indicating the synergistic ef-
fect of the N with Pd NPs also enhanced the catalytic ac-
tivity. In addition, the Pd-Co/MIL-101 showed a bit lower
activity than Pd/MIL-101, as indicated by the relatively
lower conversion (∼29%) after 90 min of reaction, maybe
due to its large particle size, indicating the synergistic
effect of Pd with Co was not important to the enhanced
activity. Further, it was worth noting that the Co@Pd/NC-
X with different Pd contents showed similar catalytic ac-
tivities for this reaction, indicating their excellent effi-
ciency and applicability (Figure 6b and Table S3).
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Table 1. Hydrogenation of various substituted nitroarenes cessful coverage of MOFs-derived Co NPs by ultrathin Pd
a
1
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8
9
1
1
1
1
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5
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5
6
over Co@Pd/NC catalyst.
shell. Particularly，when being used in hydrogenation of
nitrobenzene, our Co@Pd/NC showed much higher hy-
drogenation activity than MOFs supporting noble NPs.
The present work might open a new avenue for artificially
designed MOFs-templated non-noble@noble metal core–
shell nanocatalysts that could far surpass the traditional
MOFs supporting noble NPs in catalytic properties.
Entry Substrate
Product
Time
(min)
Conv.
(%)
Sel.
(%)
1
2
60
45
＞99
＞99
＞99
ASSOCIATED CONTENT
98
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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9
0
1
2
3
4
5
6
7
8
9
0
Additional SEM and TEM images, XRD patterns, BJH pore
size distributions, TG curves, XPS spectrum, etc. This mate-
rial is available free of charge via the Internet at
http://pubs.acs.org.
3
4
45
45
＞99
＞99
＞99
＞99
AUTHOR INFORMATION
Corresponding Author
5
45
＞99
＞99
*
liyw@scut.edu.cn
Notes
6
7
8
60
60
45
＞99
97
＞99
98
The authors declare no competing financial interest
ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China
(
21322606 and 21436005), the Doctoral Fund of Ministry of
98
＞99
Education of China (20120172110012), China Postdoctoral Sci-
ence Foundation (2015M572323), Fundamental Research
Funds for the Central Universities (2015ZM045, 2015ZP002,
and 2015PT004) and Guangdong Natural Science Foundation
(2014A030310445, 2013B090500027, and 10351064101000000)
for financial support.
9
45
45
94
96
＞99
＞99
1
0
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