Robust ultrafine ruthenium nanoparticles enabled by covalent organic gel precursor for selective reduction of nitrobenzene in water

Article abstract of DOI:10.1039/c8dt04717k

Metal nanoparticles (NPs) supported on nitrogen-doped porous carbon (NPC) are one type of promising heterogeneous catalysts. The tuning and understanding of metal-support interactions are crucial for the design and synthesis of highly durable and efficient heterogeneous catalytic systems. Here, we present an effective strategy to integrate ultrafine metal NPs into NPC via utilizing a covalent organic gel (COG) as the precursor for the first time. The ruthenium (Ru) NPs were uniformly dispersed in NPCs with the average size as low as 1.90 ± 0.4 nm. Irrespective of their ultrafine size, Ru NPs showed unprecedented stability and recyclability in Ru-catalyzed reduction of nitrobenzene and were greatly superior to commercial Ru/C and NPC-supported Ru NPs synthesized by the traditional post-loading method. This synthetic strategy can be extended to the synthesis of other metal or alloy NPs for a variety of advanced applications.

Full text of DOI:10.1039/c8dt04717k

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Robust ultrafine ruthenium nanoparticles enabled
by covalent organic gel precursor for selective
reduction of nitrobenzene in water†
Cite this: DOI: 10.1039/c8dt04717k
a,b
a,b
a
a
a
Hong Zhong, Yaqiong Gong,
Wenhui Liu, Bingbing Zhang, Shuangqi Hu*
b
and Ruihu Wang
*
Metal nanoparticles (NPs) supported on nitrogen-doped porous carbon (NPC) are one type of promising
heterogeneous catalysts. The tuning and understanding of metal–support interactions are crucial for the
design and synthesis of highly durable and efficient heterogeneous catalytic systems. Here, we present an
effective strategy to integrate ultrafine metal NPs into NPC via utilizing a covalent organic gel (COG) as
the precursor for the first time. The ruthenium (Ru) NPs were uniformly dispersed in NPCs with the
average size as low as 1.90 ± 0.4 nm. Irrespective of their ultrafine size, Ru NPs showed unprecedented
stability and recyclability in Ru-catalyzed reduction of nitrobenzene and were greatly superior to com-
mercial Ru/C and NPC-supported Ru NPs synthesized by the traditional post-loading method. This syn-
thetic strategy can be extended to the synthesis of other metal or alloy NPs for a variety of advanced
applications.
Received 29th November 2018,
Accepted 4th January 2019
DOI: 10.1039/c8dt04717k
rsc.li/dalton
coalescence after consecutive catalytic cycles owing to insuffi-
cient affinity between the supports and metal NPs. It is
Introduction
1
9
Supported metal nanocatalysts have attracted considerable highly desirable to develop a simple and effective methodology
attention in organic synthesis, energy-related catalysis and fine to tightly integrate metal NPs into NPCs for advanced
1
–4
chemistry.
Their performance is inevitably related to the nanocatalysis.
5
nature of the supports. The search for new supports with
interesting structures and properties is one of the priorities. such as zeolite imidazolate frameworks
Nitrogen-doped porous carbon (NPC) is one of the promising organic polymers,
Recently, solid-state pyrolysis of porous network materials,
2
2–24
and porous
2
5–28
has emerged as an attractive approach
supports because of its attractive properties such as large for the fabrication of NPCs. The introduction of precious
surface area, excellent electrical conductivity and high metal ions into these materials and subsequent pyrolysis can
6
–10
23,25
stability.
Metal post-loading is one common method to generate various NPC-supported metal NPs in situ.
1
1–15
introduce metal NPs into supports.
The interaction However, the random diffusion and uneven dispersion of
between nitrogen species and metal NPs not only helps to metal ions usually result in the blockage of pores and low
stabilize metal NPs on the supports, but also dramatically loading of metal ions in these porous precursors; the control
influences their surface chemistry, electronic structure and of the content, size and distribution of the resultant metal NPs
resultant catalytic performance through the hybridization of is still a formidable challenge. Covalent organic gel (COG) is
2
6–31
the p orbitals of nitrogen atoms with the d orbitals of tran- one promising alternative.
Its unique properties allow for
6
,16–20
sition metal ions.
achieved in tuning metal–support interactions for the develop- and metal ions,
Considerable progress has been the incorporation of a broad variety of functional molecules
2
9,31
which provides a tremendous opportunity
but to adjust the physicochemical properties of the resultant
1
6,19,21
ment of efficient heterogeneous catalytic systems,
these metal NPs are intrinsically inclined to aggregation and carbon materials and supported metal NPs. Furthermore, the
rich coordination sites in COG and its strong bonding ability
with metal ions can effectively resist the sintering of metal NPs
a
School of Environmental and Safety Engineering, North University of China,
Taiyuan 030051, China. E-mail: hsq@nuc.edu.cn
during pyrolysis, generating NPC-supported metal NPs with
uniform distribution, high dispersion and small size. Despite
these promising advantages, to the best of our knowledge, the
employment of COG-based materials as pyrolysis sacrificial
precursors for the formation of NPC-supported metal NPs has
not been reported hitherto.
b
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China.
E-mail: ruihu@fjirsm.ac.cn
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8dt04717k
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As a proof-of-concept study, herein, we chose metal ion- analyzed by GC. The black powder in aqueous phase was sep-
doped COG as a precursor and presented one type of durable arated by centrifugation and washed with water, followed by
heterogeneous catalytic system through in situ fabrication of centrifugation. The procedures were repeated twice, and the
2
9
NPC-encapsulated ruthenium NPs (Ru@NPC-1). The ultra- resultant solid was directly used for next run with the addition
fine Ru NPs showed excellent durability and stability in the of fresh nitrobenzene, N H ·H O and H O.
2
4
2
2
catalytic reduction of nitrobenzene, and the size and distri-
bution of ultrafine Ru NPs were intact after consecutive reac-
tions for eight runs. This study shows that COG is one type of
an outstanding precursor for the facile preparation of NPC-
supported ultrafine metal NPs.
Results and discussion
The synthetic routine of Ru@NPC-1 is shown in Scheme 1.
The imine bond formation reaction between benzene-1,3,5-tri-
carbohydrazide and 1,4-phthalaldehyde in the presence of
hydrochloride in DMF at 80 °C afforded acylhydrazone -based
Experimental
Synthesis of Ru@NPC-1
2
9
3
COG. Slow diffusion of aqueous RuCl solution above COG
led to dramatic color changes of black RuCl3 solution and
pale-yellow COG into a colorless solution and dark-grey gel,
respectively, suggesting the incorporation of Ru ions into COG.
The pyrolysis of the resultant mixture at 800 °C under a nitro-
gen atmosphere in situ generated Ru NPs, which were tightly
anchored on NPC. As a comparison, NPC was synthesized
through direct pyrolysis of COG under the same conditions,
which was used as a support of Ru NPs. The treatment of
1
,4-Phthalaldehyde (33.5 mg, 0.25 mmol) in DMF (2 mL) was
added to a DMF (2 mL) solution of benzene-1,3,5-tricarbohy-
drazide (42.0 mg, 0.17 mmol) and two drops of hydrochloric
acid (0.5 mol L− in DMF). The resultant mixture was heated at
1
8
0 °C for 72 h to afford pale-yellow COG. An aqueous solution
(10 mL) of RuCl (38.90 mg, 0.19 mmol) was added to above
3
COG, and the mixture was maintained at room temperature
for two weeks. The resultant dark grey gel was put into a
ceramic boat and heated to 800 °C at a heating rate of 5 °C
min under nitrogen atmosphere. After maintaining at 800 °C
for 2 h, the resultant powders were treated with 10 M aqueous
KOH solution at 100 °C for 24 h, followed by washing with
water and methanol, and dried in vacuo at 80 °C overnight to
afford Ru@NPC-1.
3
RuCl and NPC in water followed by pyrolysis at 800 °C under
a nitrogen atmosphere generated Ru@NPC-2. The elementary
analyses showed that the nitrogen contents in Ru@NPC-1 and
−
1
−1
Ru@NPC-2 were 3.93 and 3.26 mmol g , respectively, while
−1
the Ru contents were 0.23 and 0.18 mmol g , respectively, as
determined by inductively coupled plasma (ICP) analysis.
The porosities of Ru@NPC-1 and Ru@NPC-2 were
measured by nitrogen adsorption–desorption at 77 K (Fig. 1a).
The nitrogen sorption isotherm of Ru@NPC-1 exhibited a type
Synthesis of Ru@NPC-2
COG (3.89 g) was put into a ceramic boat and was heated to
8
I
pattern according to the IUPAC classification, while
00 °C at a heating rate of 5 °C min−1 under nitrogen atmo-
3
2,33
Ru@NPC-2 showed a combination of type I and IV.
The
sphere. After maintaining at 800 °C for 2 h, the obtained
powders were treated with 10 M aqueous KOH solution at
Brunauer–Emmett–Teller (BET) surface areas of Ru@NPC-1
2
−1
and Ru@NPC-2 were 336 and 289 m
g , respectively. Pore
1
(
00 °C for 24 h and then added to an aqueous solution
10 mL) of RuCl3 (38.90 mg, 0.19 mmol); the mixture was
stirred at room temperature for 24 h. The resultant solid was
washed with copious water to remove excess RuCl and then
size distribution revealed that Ru@NPC-1 possesses predomi-
nantly micropores, while the amount of micropores signifi-
cantly decreased in Ru@NPC-2 (Fig. 1b). In their powder X-ray
diffraction (XRD) patterns, we observed two diffraction peaks
at 2θ = 24.0 and 43.0°, corresponding to the (002) and (101)
3
dried in vacuo at 80 °C for 12 h. The obtained powders were
calcined again under the same conditions to afford
Ru@NPC-2.
3
4
planes of graphitic carbon (Fig. 1c). The typical peak of the
General procedures for nitroarene reduction
The reduction reaction was carried out in a high-pressure
reactor equipped with
Ru@NPC-2 or Ru@C was added to a mixture of substrate
0.5 mmol) and N H ·H O (2.5 mmol) in water (1 mL). The
a magnetic stirrer. Ru@NPC-1,
(
2
4
2
mixture was stirred at 35 °C for an appropriate time. After the
reaction was complete, the product was extracted with diethyl
ether (3 × 5 mL). Conversion and selectivity were determined
by GC.
General procedures for recyclability test
After the reduction reaction of nitrobenzene, the product was
Scheme 1 Schematic illustration for the synthesis of Ru@NPC-1 and
extracted with diethyl ether (3 × 5 mL). The organic layer was Ru@NPC-2.
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Fig. 1 (a) N
2
adsorption/desorption isotherms, (b) pore size distri-
butions, (c) XRD patterns and (d) Raman spectra for Ru@NPC-1 and
Ru@NPC-2.
Ru (101) plane at 44.0° was observed for Ru@NPC-1 and
2
3
Ru@NPC-2. In comparison with that of Ru@NPC-1, other
diffraction peaks observed for Ru@NPC-2 at 2θ = 38.1, 58.0,
6
9.1, 78.2 and 84.7° corresponded to the (100), (102), (110),
1
6
(103) and (112) planes of Ru NPs. The additional peaks for
Ru@NPC-2 might be attributed to relatively large particle size
and good crystallization of Ru NPs. The Raman spectra of
Ru@NPC-1 and Ru@NPC-2 exhibited two characteristic peaks
Fig. 2 TEM images of Ru@NPC-1 (a, b) and Ru@NPC-2 (c, d). The inset
images in a and c are size distribution of Ru NPs. (e) The HAADF-STEM
and EDX mapping images of Ru@NPC-1.
−
1
around 1353 and 1587 cm (Fig. 1d), corresponding to the D
3
5,36
and
G
bands.
D G
The I /I ratios in Ru@NPC-1 and
Ru@NPC-2 were 1.06 and 1.03, respectively, suggesting their
similar graphitization.
The morphology and microstructure of Ru@NPC-1 and
Ru@NPC-2 were investigated by scanning electron microscopy
NPs in Ru@NPC-1 were further demonstrated by high-annular
dark-field scanning TEM (HAADF-STEM) and energy-dispersive
X-ray (EDX) mapping images (Fig. 2e).
The existing state of surface compositions in Ru@NPC-1
and Ru@NPC-2 was investigated by X-ray photoelectron spec-
troscopy (XPS). As shown in Fig. 3a, Ru 3p XPS spectra present
two sets of doublet peaks, which correspond to Ru 3p3/2 and
(SEM) and transmission electron microscopy (TEM). SEM
images show that both Ru@NPC-1 and Ru@NPC-2 are com-
posed of small particles (Fig. S1†). Ru NPs in Ru@NPC-1 were
uniformly dispersed and possessed ultrafine size and narrow
size distribution with a mean diameter of 1.90 nm and a stan-
dard deviation of 0.4 nm (Fig. 2a and b). It should be men-
tioned that the size of Ru NPs was much smaller than those of
+
23,41,42
Ru 3p1/2 of Ru(δ ) and Ru(0) species.
The binding energy
peaks of Ru 3p3/2 in Ru@NPC-1 and Ru@NPC-2 were observed
at 461.8 and 462.2 eV, respectively, which were assigned to Ru
(
0) species, while Ru 3p3/2 peaks at 464.0 and 464.6 eV corre-
3
7–40
most reported Ru NPs supported on porous carbons,
+
+
sponded to Ru(δ ) species. The ratios of surface Ru(0) to Ru(δ )
which probably resulted from smooth diffusion and homo-
geneous distribution of Ru ions in COG as well as the sintering
resistance of Ru NPs by gel network during pyrolysis. In sharp
contrast, considerable agglomeration and uneven distribution
of Ru NPs were observed in Ru@NPC-2 (Fig. 2c and d). These
results clearly indicated that the post-loading of metal NPs to
NPC can lead to insufficient anchoring of NPs on the supports
when compared with in situ integration of metal NPs into
NPC. The high-resolution TEM (HRTEM) images of Ru@NPC-1
and Ru@NPC-2 show the same interval of 0.204 nm between
the adjacent lattice fringes, which corresponds to the (101)
2
3
plane of Ru NPs. This result was consistent with that of XRD
Fig. 3 (a) Ru 3p XPS spectra of Ru@NPC-1 and Ru@NPC-2; (b) N 1s XPS
analyses. The ultrafine size and uniform distribution of Ru spectra of Ru@NPC-1, Ru@NPC-2 and NPC.
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in Ru@NPC-1 and Ru@NPC-2 were 1.12 and 1.41, respectively, due to the segmental coverage of Ru active sites by NPC. For
as determined by the ratio of their relative peak areas. The Ru comparison, commercial Ru@C was also tested under the same
3p3/2 binding energy peak of Ru(0) species in Ru@NPC-1 nega- conditions, and a complete conversion of nitrobenzene was
tively shifted by 0.40 eV in comparison to that of Ru@NPC-2, achieved in 2.5 h, which was inferior to that of Ru@NPC-2. It
which indicated a stronger interaction between Ru(0) species should be mentioned that the selectivity of aniline remained
and the support in Ru@NPC-1. To further confirm the inter- above 99% in the entire catalytic process for these catalytic
actions, N 1s XPS spectra of NPC, Ru@NPC-1 and Ru@NPC-2 systems. Control experiments were also conducted either in the
were also investigated (Fig. 3b). In the high-resolution N 1s absence of Ru catalysts or in the presence of NPC, and only
XPS spectrum of NPC, the presence of pyridinic N (398.1 eV), trace amount of nitrobenzene was reduced to aniline.
pyrrolic N (399.3 eV) and graphitic N (400.7 eV) could be clearly
In heterogeneous catalytic systems, recyclability and stabi-
26
identified; these binding energy peaks in Ru@NPC-2 posi- lity are important parameters besides activity. As shown in
tively shifted by 0.31, 0.96 and 0.32 eV, respectively, while more Fig. 4b, Ru@NPC-1 can be used for at least eight runs with
positive shifts by 0.35, 0.98 and 0.38 eV, respectively, were complete conversion of nitrobenzene and sole selectivity of
observed for Ru@NPC-1. These shifts toward higher binding aniline in the hydrogenation reaction, while the conversions of
energies mainly originated from the coordination of NPC nitro- nitrobenzene for Ru@NPC-2 decrease to 84 and 26% in the
6
gen atoms with Ru, which led to electron donation from NPC fourth and fifth runs, respectively. In contrast, Ru/C provided
to Ru species. The resultant electron-rich Ru(0) species were 49% and 17% conversions of nitrobenzene in the second and
favorable for the activation of the substrate molecules in the third runs, respectively. The outstanding heterogeneous behav-
catalytic reactions. The more positive shift of N 1s peaks in ior of Ru@NPC-1 was also verified by a hot filtration experi-
Ru@NPC-1 further indicated stronger interactions between Ru ment. After the reaction was carried out for 1 h, the catalyti-
species and NPC than that observed for Ru@NPC-2, which was cally active species were quickly removed by filtration, and the
consistent with the results of Ru 3p XPS analysis.
reaction of the filtrate was continued for additional 2 h; both
Aromatic amines are important intermediates in the indus- conversion and selectivity showed negligible variation (entries
trial synthesis of pigments, pharmaceuticals and agricultural 1 and 2, Table 1). After the first run of the reduction reaction,
4
3,44
products.
One of the typical methods for their fabrication the filtrate was analyzed by ICP, and it was found that only
is selective reduction of nitroarenes over metal NPs immobi- 0.11 ppm Ru was leached into the solution from Ru@NPC-1.
4
3–47
lized on solid supports.
The main challenge in hetero- However, when Ru@NPC-2 was used under the same con-
geneous hydrogenation is to achieve both high catalytic activity ditions, 1.53 ppm Ru was leached into the solution owing to
4
4
and high selectivity towards the nitro group, especially in the relatively weak interactions between NPC and Ru NPs.
presence of sensitive groups. In addition, the hydrogenation of The stability of Ru NPs in the catalytic reaction was further
nitroarenes often requires high pressure of hydrogen gas examined. After Ru@NPC-1 and Ru@NPC-2 were used for con-
4
4
and/or the use of environmentally harmful organic solvents.
secutive nitrobenzene hydrogenation for eight runs and five
Ru is known to possess low price and high selectivity in hydro- runs, respectively, the resultant powders containing Ru species
genation reaction, but the activity is relatively low when com- were isolated and denoted as Ru@NPC-1–8run and
2
3
pared with that of palladium. In our catalytic systems, hydra- Ru@NPC-2–5run. SEM images show that the morphologies of
zine monohydrate was selected as a safe and convenient Ru@NPC-1–8run and Ru@NPC-2–5run are consistent with
4
4
source of hydrogen with released N
2
as the only side-product.
those before hydrogenation reaction (Fig. S2†). TEM images
The catalytic performance of Ru@NPC-1 and Ru@NPC-2 was show that Ru NPs in Ru@NPC-1–8run are still uniformly dis-
initially evaluated by the reduction of nitrobenzene in water. persed over the support, and their average size is well retained
As shown in Fig. 4a, Ru@NPC-1 afforded complete conversion at 1.90 ± 0.4 nm (Fig. 5a and b), suggesting outstanding dura-
of nitrobenzene to aniline in
3 h, while the use of bility of Ru@NPC-1 in the heterogeneous catalytic reaction. In
Ru@NPC-2 gave complete conversion in 2 h under the same sharp contrast, an apparent agglomeration of Ru NPs was
conditions. The slightly low activity in Ru@NPC-1 is probably observed in Ru@NPC-2–5run (Fig. 5c and d). These results
revealed that COG is a promising precursor for the fabrication
of supported metal NPs.
To extend the application scope of this catalytic system, the
hydrogenation reactions of various nitroarenes were examined
using hydrazine monohydrate as a reductant in the presence
of Ru@NPC-1. As shown in Table 1, the substrates with elec-
tron-donating groups, such as 1-methyl-4-nitrobenzene,
4-nitrophenol, 1-methoxy-4-nitrobenzene and 4-nitroaniline,
provide full conversion and more than 99% selectivity to the
corresponding aromatic amines (entries 3–8). The steric hin-
drance at ortho-, meta- and para-positions of nitrotoluene has
no significant effects on the catalytic reactions (entries 3–5).
The substrates with electron-withdrawing groups, such as
Fig. 4 (a) The kinetic curve and (b) recyclability for Ru@NPC-1,
Ru@NPC-2 and Ru@C in the reduction reaction of nitrobenzene.
Reaction conditions: Nitrobenzene (0.5 mmol), [Ru] (1 mol%), N
2.5 mmol), H O (1 mL) at 35 °C.
2 4 2
H ·H O
(
2
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a
Table 1 Nitroarenes reduction in the presence of Ru@NPC-1
1-chloro-4-nitrobenzene, 1-chloro-2-nitrobenzene and 1-(4-
nitrophenyl)ethan-1-one, also afforded full conversion and
more than 99% selectivity to the target products (entries 9–11).
Notably, when 2-nitrochlorobenzene and 4-nitrochlorobenzene
were used as substrates, only 2-chloroaniline and 4-chloroani-
line were obtained, and no dehalogenation products of aniline
were detected, which may be attributed to the intrinsic selecti-
vity of Ru NPs (entries 9 and 10). 4-Nitroacetophenone also
underwent total conversion with >99% selectivity to the corres-
ponding aniline derivative, and no carbonyl reduced product
was detected (entry 11), which was different from the results of
a
b
b
Entry
Substrate
Product
Conv. (%)
Select. (%)
c
1
2
3
4
35
36
>99
c
>99
>99
>99
>99
>99
5
6
7
8
9
1
>99
>99
98
>99
>99
>99
>99
>99
>99
4
6
reported Ru NPs. Interestingly, complete conversion of 1,3-
dinitrobenzene to 1,3-diaminobenzene was also achieved in
3
h (entry 12). The catalytic system was also active for hydro-
genation of 3-nitropyridine, and the target product was
obtained in a quantitative yield (entry 13).
96
On the basis of the above-mentioned observations, the for-
mation of ultrafine Ru NPs and remarkably durability of
Ru@NPC-1 in the reduction of nitrobenzene were mainly
ascribed to the unique structure merits of COG. Ru ions could
be homogeneously distributed in the gel network through the
>99
>99
0
1
3
diffusion of aqueous RuCl solution in COG. The acylhyrazone
1
>99
>99
units possessed strong coordination ability toward Ru ions. Ru
cations were reduced to form Ru NPs during pyrolysis under
an inert atmosphere. Meantime, the host backbone and
embedded guest molecules in COG served as carbon and nitro-
gen sources for the formation of NPC. As demonstrated in the
XPS spectrum of Ru@NPC-1, high content of nitrogen atoms
in NPC could modify geometrically and electronically in situ
generated Ru NPs, endowing the obtained materials with
superior catalytic activity. NPC could effectively resist the sin-
tering of Ru NPs during pyrolysis, generating well-dispersed
ultrafine Ru NPs even at the pyrolysis temperature as high as
1
1
2
3
>99
>99
>99
>99
a
Reaction conditions: Nitrobenzene (0.5 mmol), Ru@NPC-1 (1 mol%),
H ·H O (2.5 mmol), H O (1 mL), 35 °C, 3 h. GC yield. Filtration
2 4 2 2
b
c
N
experiment.
800 °C. As shown in Fig. 6, when compared with metal NPs in
Ru@NPC-2 synthesized using the traditional post-loading
method, Ru NPs in Ru@NPC-2 possessed large particle size
and uneven size distribution despite their nitrogen content,
Fig. 5 TEM images for (a, b) Ru@NPC-1–8run and (c, d)
Ru@NPC-2–5run.
Fig. 6 Schematic illustration for stability of Ru NPs during consecutive
catalytic cycles in (a) Ru@NPC-1 and (b) Ru@NPC-2.
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Ru loading and graphite degree being almost the same the Strategic Priority Research Program of the Chinese
because Ru NPs were stabilized mainly through the coordi- Academy of Sciences (XDB20000000).
nation interaction of nitrogen atoms in NPC; the interaction
was not strong enough to suppress the aggregation and leach-
ing during catalytic reactions. Thus, the catalytic activity of
Ru@NPC-2 gradually decreased after the consecutive catalytic
Notes and references
cycles. In sharp contrast, ultrafine Ru NPs in Ru@NPC-1
possessed high surface energy, but they were tightly anchored
on NPC; the interactions between NPC and Ru NPs
were much stronger than that observed for Ru@NPC-2, which
effectively inhibited the aggregation and coalescence during
catalytic reaction. Impressively, the ultrasmall size and uniform
distribution of Ru NPs were almost the same after consecutive
catalytic cycles. The integration of Ru NPs into NPC from the
COG precursor by the in situ growth strategy was beneficial for
enhancing the stability of the catalytic systems. It should be
mentioned that the protocol for the synthesis of ultrafine Ru
NPs exhibited excellent generality and could be extended to the
fabrication of other metallic NPs. The treatment of COG with
aqueous PdCl2 and K PtCl solutions under the same con-
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Conclusions
We developed a facile method for the in situ synthesis of
robust NPC-supported metal NPs through the use of COG con-
taining metal ions as the precursor for the first time.
Irrespective of the ultrafine size and narrow size distribution
of Ru NPs, the unprecedented catalytic performance in terms
of stability and reusability was displayed in the selective
reduction of nitroaromatic compounds. The ultrafine Ru NPs
still maintained their uniform dispersion, identical particle
size and high catalytic activity after consecutive cycles for eight
runs. The outstanding performance was mainly attributed to
intensive metal–support interactions when compared with that
of Ru NPs supported by NPC obtained by post-loading. This
study has demonstrated the unique advantages of COG for the
synthesis of robust ultrafine Ru NPs over NPC, and the pro-
posed protocol holds great promises for the general synthesis
of a broad range of ultrafine metal NPs supported by NPC for
various heterogeneous catalyses.
1
2 P. Zhang, Y. Gong, H. Li, Z. Chen and Y. Wang, Nat.
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Acknowledgements
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