Size-controllable palladium nanoparticles immobilized on carbon nanospheres for nitroaromatic hydrogenation

Article abstract of DOI:10.1039/c3ta00159h

In this paper, monodisperse Pd nanocrystals were immobilized on previously reported carbon nanospheres via in situ adsorption and reduction. In this protocol, no excess reductant and capping reagents were necessary, which made the surface of the as-prepared nanocatalysts very clean. Using sodium tetrachloropalladate(ii) as the metal precursor yielded palladium nanocrystals with a size around 5 nm regardless of the metal loading, while the use of palladium chloride resulted in a size increase to 18.1 nm. Moreover, the additives used during preparation have been proven to be of great importance in controlling the average particle size. It was suggested that the pattern of the adsorbed palladium ions or the surface environment of support was greatly influenced. Strong adsorption of the palladium ions on the carbon spheres led to a decrease in size. The nanocrystals exhibited excellent catalytic activity for transfer hydrogenation under ambient conditions. The conversion was 83.1% to 100% for several nitroaromatics with moderate to excellent selectivity. More importantly, these nanocatalysts are promising for renewable catalysis owing to their sustainable support, green catalyst fabrication and ease of handling.

Full text of DOI:10.1039/c3ta00159h

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Materials Chemistry A
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Size-controllable palladium nanoparticles immobilized
on carbon nanospheres for nitroaromatic
hydrogenation†
Cite this: J. Mater. Chem. A, 2013, 1,
3783
*
Yong-Ming Lu, Hai-Zhou Zhu, Wei-Gu Li, Bo Hu and Shu-Hong Yu
In this paper, monodisperse Pd nanocrystals were immobilized on previously reported carbon nanospheres
via in situ adsorption and reduction. In this protocol, no excess reductant and capping reagents were
necessary, which made the surface of the as-prepared nanocatalysts very clean. Using sodium
tetrachloropalladate(II) as the metal precursor yielded palladium nanocrystals with a size around 5 nm
regardless of the metal loading, while the use of palladium chloride resulted in a size increase to
18.1 nm. Moreover, the additives used during preparation have been proven to be of great importance
in controlling the average particle size. It was suggested that the pattern of the adsorbed palladium
ions or the surface environment of support was greatly influenced. Strong adsorption of the palladium
ions on the carbon spheres led to a decrease in size. The nanocrystals exhibited excellent catalytic
activity for transfer hydrogenation under ambient conditions. The conversion was 83.1% to 100% for
several nitroaromatics with moderate to excellent selectivity. More importantly, these nanocatalysts are
promising for renewable catalysis owing to their sustainable support, green catalyst fabrication and
ease of handling.
Received 11th January 2013
Accepted 14th January 2013
DOI: 10.1039/c3ta00159h
www.rsc.org/MaterialsA
nanospheres, graphene, etc.) as catalyst supports which have
exhibited unique or unusual behavior. For example, by intro-
Introduction
Nowadays the desire for green and sustainable chemistry is
attracting more and more attention.1 Biomass, as the most
common renewable feedstock, has been proven to be a qualied
carbon raw material. Many synthetic techniques have been
reported for converting biomass into amorphous, carbona-
ceous, porous, or crystalline carbon materials with different
sizes, shapes, and surface functional groups. Amongst various
methods, hydrothermal carbonization (HTC) processing of
biomass is used extensively to synthesise an enormous number
of novel carbonaceous nanomaterials.2 The low-temperature
ducing Rh particles into the cavities of carbon nanotubes, Bao
et al.4 discovered that the catalytic activity for the conversion of
CO and H2 to ethanol was greatly enhanced. The overall
reaction rate inside the nanotubes was ten times more than
that outside the nanotubes. Motoyama et al.5 reported three
types of carbon nanobers decorated with platinum and
palladium nanoparticles as catalysts for the hydrogenation of
nitroarenes. High turnover numbers and chemoselectivity were
obtained under mild reaction conditions. Makowski et al.6
reported the catalytic properties of Pd nanoparticles supported
on hydrophilic carbon spheres. An optimal selectivity and good
conversion were realized in hydrogenating phenol to cyclo-
hexanone. Recently, Lu and coworkers synthesized highly
uniform carbon nanospheres on the basis of benzoxazine
chemistry, and palladium particles loaded on such nano-
spheres as a catalyst exhibited a high catalytic activity, great
reusability, and regeneration ability in a selective benzyl
alcohol oxidation.7
ꢀ
HTC process, carried out around 200–250 C, has been widely
used as a more environmentally friendly route, which may
synthesise carbonaceous materials with higher reactivities,
microporosities or mesoporosities, and controllable morphol-
ogies. These features make the as-synthesized materials very
useful in many areas such as drug delivery, energy storage,
water purication, gas sensors, and catalysis.3
Recently there has been an increasing interest in dispersed
carbonaceous
nanomaterials
(nanobers,
nanotubes,
It is well known that the amount of metal, the size of the
particles, the preparation method, and the support composition
play crucial roles in the performance of heterogeneous cata-
lysts.8 By employing transition metal nanoparticles of uniform
size and shape, the reaction activities and product selectivities
of many heterogeneous catalytic reactions could be greatly
inuenced.9–11 Meanwhile the catalyst support provides the
Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical
Sciences at Microscale, The School of Chemistry and Materials, University of Science
and Technology of China, Hefei 230026, PR China. E-mail: shyu@ustc.edu.cn; Fax:
+86-551-3603040; Tel: +86-551-3603040
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ta00159h
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solution to separate the catalyst from the reactant/product and
prevent activity loss by metal leaching. Otherwise, the size and
shape of the supported metal particles can be signicantly
inuenced by the carrier due to different interactions. For
example, Ni nanoparticles were supported on HZSM-5 and
Al2O3-HZSM-5 with average sizes of 35 and 8.8 nm respectively.12
The catalytic activity for phenol hydrogenation was up to ve
times higher for Ni/Al2O3-HZSM-5. Ni leaching was also
observed to be much slower from Ni/Al2O3-HZSM-5.
Recycle analysis
Aer the reaction period was over, the reaction mixture was
cooled to room temperature and centrifuged. The solid catalyst
was collected and fresh reactant was added to react. To analyse
Pd leaching during the reaction, the mixed solvent of the
centrifugate was evaporated under reduced pressure and the
residue was dissolved in aqua regia (concentrated HCl : con-
centrated HNO3 ¼ 3 : 1, v/v) and H2O. This solution was then
analysed by ICP-AES.
We have previously reported well-dened carbonaceous
nanomaterials from glucose by the HTC process.13,14b These
materials have different diverse structures enriched with active
functional groups. In this paper, we report the facile synthesis
of carbon nanosphere based palladium catalysts by in situ
adsorption of metal ions and reduction by surface groups. Such
a protocol not only makes the catalyst construction simple and
green, but also easily provides the metal nanocrystals with clean
surfaces. The as-prepared Pd nanoparticles immobilized on
carbon nanospheres are expected to exhibit excellent catalytic
activity for the transfer hydrogenation of nitroaromatics.
Characterization
X-ray powder diffraction patterns (XRD) of the product were
obtained on a Japan Rigaku DMax-gA rotation anode X-ray
diffractometer equipped with graphite monochromatized Cu KR
radiation (l ¼ 1.54178 A). X-ray photoelectron spectroscopy (XPS)
˚
was conducted on an ESCALab MKII X-ray photoelectron spec-
trometer using an Mg Ka radiation excitation source. Scanning
electron microscope (SEM) measurements were carried out
using a Zeiss Supra 40 microscope. Transmission electron
microscope (TEM) photographs were taken on a Hitachi model
H-800 transmission electron microscope at an accelerating
voltage of 200 kV. The catalytic process performance was directed
by GC (Agilent 7890A series) equipped with a FID detector and a
HP-5 column (30 mm ꢁ 0.32 mm ꢁ 0.25 mm). The structure of the
product was conrmed by comparison with a standard sample
and by GC-MS (Agilent GC-MS 6890/5973). Inductively coupled
plasma atomic emission spectroscopy (ICP-AES) was recorded
using an Atomscan Advantage (Thermo Jarrell Ash Corporation,
U.S.A.) instrument. UV-vis spectra were recorded with a Shi-
madzu UV-2550 spectrophotometer at room temperature.
Experimental section
All chemicals are of analytical grade and were used as received
without further purication.
Synthesis of carbonaceous nanospheres
8 g of glucose were added to 80 mL of deionized water with
stirring to form a clear solution. Aer that, the solution was
transferred into a Teon-lined 100 mL autoclave, heated to
180 ꢀC and kept at that temperature for 8 h. Aer cooling to
room temperature, the product was centrifuged and washed
several times with deionized water and ethanol to remove other
impurities and nally freeze-dried in a vacuum.
Results and discussion
The carbon nanospheres (CSs) were prepared according to a
previously reported method.14 These carbonaceous materials
exhibit uniform spherical shape and show excellent dis-
persibility, which favors the subsequent catalyst fabrication
(Fig. 1a). Due to the numerous surface functional groups con-
sisting of carbonyl and hydroxy moieties which demonstrate
strong reducibility and adsorption properties, palladium
precursors can be easily deposited and reduced in situ. Briey,
the catalyst was prepared by mixing sodium tetra-
chloropalladate(II) with aqueous carbon nanospheres and heat-
ing to reux for 5 h. ICP analysis of the sample revealed that the
actual Pd content was 1.36 wt%. The morphology of the Pd/CSs
was characterized by TEM (Fig. 1b) and HRTEM (Fig. 1c). From
the images, the spherical shape and diameter of the spheres were
unaffected compared to CSs. Pd nanoparticles were clearly
Deposition of Pd nanocrystals on carbonaceous nanospheres
As-synthesized carbonaceous nanospheres (CSs) were dispersed
in deionized water with the aid of sonication and reuxed in an
oil bath for 30 minutes, then a solution of a palladium salt was
added. The mixed solution was further reuxed in an oil bath
for another 5 hours. Aer cooling to room temperature the
product was centrifuged (8000 rpm, 10 min) and washed several
times with deionized water. The nal product was freeze-dried
in a vacuum. In order to introduce the additives, they were
rstly mixed with the metal salt solution and then the mixed
solution was added as a source of Pd.
Transfer hydrogenation of nitroaromatics
The hydrogenation of nitroaromatics was examined over all anchored on the spheres and a uniform dispersity was also
ꢀ
catalysts at 80 C. In a typical process, nitroaromatics and Pd/ observed. The average size of these Pd nanoparticles was
CSs with a mole ratio of 2000 were dissolved in a mixed solvent approximately 5.4 nm. The enlarged HRTEM image is also shown
of ethanol and water (v/v, 1 : 1). Then 4 equivalents of hydrazine in Fig. 1c, clearly displaying lattice fringes of about 0.26 nm
hydrate were added and stirred for a certain time. The reaction which is consistent with the (111) plane of the palladium nano-
was monitored and analyzed by characterizing the reaction crystals. The X-ray diffraction (XRD) pattern shown in Fig. 1d
mixture using gas chromatography.
illustrates three characteristic diffraction peaks of the face
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The Pd source had a pronounced effect on the supported
nanoparticle size. Sodium tetrachloropalladate(II) with a better
solubility than palladium chloride in aqueous media, led to a
smaller average Pd particle size. Meanwhile, better reaction
conversion and selectivity was obtained (Table 1, entries 1, 2,
3 vs. 4), which might be ascribed mainly to an increase in the
number of active sites for small nanoparticles. From Table 1,
increasing the Pd loading from 0.84 to 1.36 and 3.06 wt% had a
slight effect on the average particle sizes (ESI, Fig. S1–S3†) and
reaction results. Palladium nanoparticles with an average size
of about 5.4 nm had the best hydrogenation reaction activity
and selectivity (Table 1). The recycling study was also carried out
with 1.36 wt% Pd/CSs. Aer each reaction, the catalyst was
easily recovered by centrifugation and reused directly. In this
case, the yield of aniline was 100%, 92.3% and 81.8% for three
cycles, which indicated that the catalyst remained active under
the reaction conditions. ICP analysis of the reaction solution
revealed that 3.4% Pd was leached during each cycle, and the
tendency of Pd nanoparticles to grow into large crystallites was
also observed in the TEM image of the recovered catalyst (ESI,
Fig. S4†). It is noteworthy that no inert atmosphere was required
in this protocol and the same result was obtained when the
reaction was conducted under a nitrogen atmosphere.
It is interesting that the use of additives during the catalysts
preparation had an important impact on the reaction activity
and particle size. The addition of sodium chloride (NaCl)
resulted in a size reduction from 17.5 nm to 7.3 nm (ESI, Fig. S5
and S6†) and a signicant activity improvement (Table 1, entry
6). However, upon the addition of hydrochloric acid (HCl), a
minimal impact on nanoparticle size (ESI, Fig. S7†) and similar
activity improvement were observed (Table 1, entry 5). It has
been reported that larger palladium nanoparticles supported on
g-Fe2O3 had a higher catalytic activity for hydrogenation and the
superior activity was considered to be caused by electronic and
adsorptive characteristics.18 It is also well known that the
physical and chemical properties of carbon materials could be
tailored, and the performance of catalysts could be optimized by
modifying the surface functional groups on the carbon
supports.19 For instance, Mondal et al. reported that the cata-
lytic activity of ethylene hydrogenation increased aer acid
treatment of carbon microspheres but the Pd metal sintered
Fig. 1 (a) SEM image of carbon nanospheres; (b) TEM and (c) HRTEM images
of carbon nanospheres deposited with palladium nanoparticles (1.36 wt%); (d)
X-ray diffraction pattern of Pd/CSs.
centered cubic crystalline phase of palladium, consistent with
the HRTEM measurements.
X-ray photoelectron spectroscopy (XPS) was employed to
investigate the surface chemical composition and electronic
structure of the nanocrystals. The survey spectrum presented in
Fig. 2a conrms the existence of Pd, O and C. Meanwhile
abundant C and O species were revealed to indicate the pres-
ence of versatile oxygen-containing functional groups on the
materials surface. In the high resolution Pd 3d spectrum
(Fig. 2b) characteristic peaks are observed with binding energies
(BE) of 335.6 eV (Pd 3d5/2) and 340.8 eV (Pd 3d3/2), ascribed to
metallic Pd similar to that seen in Pd/C or Pd powder.15 The
second set of doublets, which was observed at a BE of approx-
imately 0.8 eV higher than that for Pd(0), could be attributed to
the Pdd+ chemical state.
Aromatic amines, which are key intermediates for the
synthesis of dyes, pharmaceuticals, and agricultural chemicals,
are oen produced by the catalytic reduction of nitroaromatics
over supported metal catalysts.16 Recently, attempts to make the
inexpensive Pd catalysts more selective have drawn increasing
attention.17 In addition, mild reaction conditions such as cata-
lytic transfer hydrogenation are always in demand. Herein, the
transfer hydrogenation of nitrobenzene was chosen as a model
reaction to evaluate the catalytic activity of the Pd/CSs
nanocatalysts.
Table 1 Transfer hydrogenation of nitrobenzene catalyzed by carbon nano-
sphere supported palladium particlesa
Pd loading Mean size
Entry Pd source
[wt%]
[nm]
Conv.b [%] Sel.b [%]
1
2
3
4
5
6
Na2PdCl4
Na2PdCl4
Na2PdCl4
PdCl2
0.84
1.36
3.06
2.20
4.9
5.4
5.3
17.5
18.1
7.3
100
100
100
76.7
100
100
95.6
100
100
70.0
90.0
95.2
PdCl2 + HClc 2.37
PdCl2 + NaClc 2.45
a
Reaction conditions: nitrobenzene (1 mmol), hydrazine hydrate
(4 mmol), Pd (5 ꢁ 10ꢂ4 mmol), solvent (ethanol/H2O ¼ 1 mL : 1 mL),
b
c
air, 80 ꢀC, 150 min. Determined by GC analysis. HCl or NaCl was
added to PdCl2 solution before depositing on carbon nanospheres.
Fig. 2 (a) XPS survey spectrum and (b) high resolution XPS spectrum of Pd 3d for
1.36 wt% Pd/CSs.
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under the reaction conditions.20 In our carbon nanosphere
supported palladium catalysts, versatile oxygen-containing
functional groups (mainly CHO and OH moieties) are enriched
on the surface, which produced the high dispersion of Pd and
might enhance the metal–support interactions to realize high
reaction activities.
The additive effect on the particle sizes was also investigated
for the sodium tetrachloropalladate precursor. The results are
presented in Fig. 3. It can be seen that a high dispersion of Pd
particles was achieved with a Na2PdCl4 precursor accompanied
by additives. There was no obvious change in the particle size in
the presence of a HCl additive which resulted in Pd nano-
particles with an average size of about 5.2 nm. However upon
the addition of NaCl a notable size increase to 10.3 nm was
detected. In order to illustrate the above-mentioned size
changes, UV-vis absorption spectroscopy was carried out to
identify the structure and pattern of palladium ions in the
absence or presence of additives. Despite the use of different Pd
precursors, three absorption bands appeared at ꢃ225, 280 and
Fig. 4 (a and b) UV-vis spectra of PdCl2 in aqueous (I), hydrochloric acid (II) and
sodium chloride (III) solutions; (c and d) UV-vis spectra of Na2PdCl4 in aqueous (IV),
hydrochloric acid (V) and sodium chloride (VI) solutions.
475 nm (Fig. 4, III and VI) upon NaCl addition. These are
2ꢂ
assigned to the bands of a PdCl4 complex.21 The existence of obtained. Thus the adsorption should be greatly weakened. The
this Pd anion made adsorption to the negatively charged carbon TEM image (ESI, Fig. S8†) clearly shows that no Pd crystals were
spheres more difficult. Moreover, chloride adsorption compe- discovered on the carbon spheres. As seen from the UV-vis
2ꢂ
tition with the PdCl4 complex cannot be neglected.22 Thus spectra for the PdCl2 precursor (Fig. 4, I and II), more positive
+
large Pd nanocrystals were deposited. The UV-vis spectra of Pd ions appeared which were assigned to the PdCl(H2O)3 and
aqueous Na2PdCl4 solution and the solution in the presence of PdCl2(H2O)2/PdCl(H2O)3 complexes respectively. However,
+
HCl showed two characteristic absorption bands centered at PdCl2 could not be completely dissolved under these condi-
ꢃ208 and 237 nm (Fig. 4, V and IV). These could be ascribed to tions. This might inhibit the adsorption of the Pd precursors on
the ligand-to-metal charge transfer bands of the PdCl2(H2O)2/ the surface of the carbon nanospheres. So the largest Pd
+
PdCl(H2O)3 and PdCl2(H2O)2 complexes,21 which exhibited nanocrystals were formed.
2ꢂ
stronger adsorption with carbon spheres than the PdCl4
Following the successful hydrogenation of nitrobenzene by
complex. Although the addition of HCl brought about a more Pd/CSs, nitroaromatics bearing different functional groups
negatively charged Pd complex, the introduction of protons also were also studied and the results are summarized in Table 2. As
produced more positively charged supports. For comparison, can be seen from Table 2, nitroaromatics were smoothly
the Na2PdCl4 precursor dissolved in NaOH solution was also reduced by transfer hydrogenation. Vinyl and carbonyl substit-
studied for deposition on carbon nanospheres. In this case, uents were highly tolerant and showed high selectivity during
negatively charged Pd anions and carbon spheres were both the reactions (entries 2 and 3). The position of the substituents
had no obvious effect on the reaction activity or selectivity
(entries 6–10). Unfortunately, 4-nitrobenzonitrile gave only
52.4% selectivity to the corresponding aminobenzonitrile and
the hydrolysis product was detected. Solvent, reaction temper-
ature and amount of hydrazine hydrate were screened to ensure
that they had no effect on the selectivity improvement. Gener-
ally, palladium catalysts are known to show a high tendency
toward dehalogenation.23 Efforts to suppress this side reaction
were extensively made by addition of inhibitors, poisoning,
alloying and support modulating.5,18,24 For example, diphenyl
sulde modied Pd/C gave a 91% yield for 4-chloroaniline
synthesis using molecular hydrogen.24a Hydrogenolysis of C–Cl
was obviously inhibited from 35.7% to 3.4% with Sn4+ modi-
cation on a PVP-Pd/Al2O3 catalyst.24b A Pd/g-Fe2O3 nano-
composite catalyst was partially reduced to adjust the surface
electronic and adsorptive characteristics, by which dehaloge-
nation was reduced to 3.6%.18 Without additives and pretreat-
ment, the as-prepared Pd/CSs produced only 6.5%
Fig. 3 TEM images and size distribution profile of carbon nanosphere supported
dechlorinated product by a more exible and easy to handle
catalytic transfer hydrogenation process.
Pd catalysts derived from a Na2PdCl4 precursor in the presence of (a and b)
hydrochloric acid and (c and d) sodium chloride.
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Table 2 Transfer hydrogenation of various nitroaromatics with 1.36 wt% Pd/CSs
catalyst under ambient conditionsa
It should be noted that the metal content played a crucial
role for the product distribution in the transfer hydrogenation
of 4-chloronitrobenzene. Increasing metal loading gradually
favored the dechlorination to produce as much as 23.5% aniline
using 3.06 wt% Pd/CSs while the formation of the intermediate
azoxy and azo compounds was reduced (Fig. 5). Consequently,
1.36 wt% Pd/CSs is the most selective among as-synthesized
catalysts.
Entry
R
T (h)
Conv.b (%)
Sel.b (%)
Conclusions
1c
2c
3
H
2.5
2.5
0.5
6
1.5
24
24
4
100
100
100
100
100
83.1
95.4
100
97.9
92.9
100
100
4-Vinyl
3-CHO
4-CH3
4-Cl
4-OH
3-OH
4-NH2
3-NH2
2-NH2
In summary, monodisperse and uniform palladium nano-
particles can be easily deposited on biomass-based carbon
nanospheres with controllable sizes. The palladium precursor,
additive and metal loading have a great inuence on the size of
loaded Pd nanoparticles and their catalytic performance.
Compared with other Pd catalysts, the Pd/CSs showed good
activities and selectivities for nitroaromatic hydrogenation in
the absence of additives or special pretreatment. These results
demonstrate that it is possible to prepare inexpensive Pd
nanocatalysts with good selectivity by an easy, green and
sustainable method.
95.2
4
72d
5e
6
86.2
100
7
8
9
10
84.4d
100
10
10
90.6d
100
a
Reaction conditions: substrate (1 mmol), hydrazine hydrate (10 mmol),
1.36 wt% Pd (5 ꢁ 10ꢂ4 mmol), solvent (ethanol/H2O ¼ 1 mL : 1 mL), air.
b
Calculated by gas chromatography. c 4 mmol hydrazine hydrate. d The
main byproducts were ArPhN]NPhAr and ArPhN]NOPhAr. e 20 mmol
hydrazine hydrate and 1 mL anhydrous ethanol as solvent.
Acknowledgements
Several reaction pathways have been proposed to understand
the reduction of nitroaromatic compounds to their corre-
sponding aniline.25 In this protocol, the formation of azoxy and
azo compounds, which was achieved in up to 25.5% yield for the
reduction of 4-chloronitrobenzene aer 30 min, indicated the
condensation route in the protocol. However, the conversion of
azoxy/azo intermediates to nal products was very sluggish in
spite of the amount of hydrazine hydrate. For instance, the
yields of 4-chloroaniline and its corresponding azoxy/azo
intermediates were 68.6% and 25.5% aer reaction for 30
minutes, respectively, while it only changed to 86.2% and 7.3%
respectively, aer 1.5 hours.
We acknowledge the funding support from the National Basic
Research Program of China (Grant 2010CB934700), the
National Natural Science Foundation of China (Grants
91022032, 912271032), the Chinese Academy of Sciences (Grant
KJZD-EW-M01-1), and the Principal Investigator Award by the
National Synchrotron Radiation Laboratory at the University of
Science and Technology of China.
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3788 | J. Mater. Chem. A, 2013, 1, 3783–3788
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