Preparation of nickel nanoparticles with different sizes and structures and catalytic activity in the hydrogenation of p-nitrophenol

Article abstract of DOI:10.1039/b9nj00657e

Spherical nickel nanoparticles with different sizes and different crystal structures were prepared by reducing nickel oxalate with hydrazine hydrate in the presence of PEG 200, PEG 6000, Tween 40, Tween 80, and sodium dodecyl sulfonate as organic modifier

Full text of DOI:10.1039/b9nj00657e

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www.rsc.org/njc | New Journal of Chemistry
Preparation of nickel nanoparticles with different sizes and structures
and catalytic activity in the hydrogenation of p-nitrophenol
Aili Wang, Hengbo Yin,* Min Ren, Huihong Lu, Jinjuan Xue and Tingshun Jiang
Received (in Victoria, Australia) 10th November 2009, Accepted 4th January 2010
First published as an Advance Article on the web 12th February 2010
DOI: 10.1039/b9nj00657e
Spherical nickel nanoparticles with different sizes and different crystal structures were prepared by
reducing nickel oxalate with hydrazine hydrate in the presence of PEG 200, PEG 6000, Tween 40,
Tween 80, and sodium dodecyl sulfonate as organic modifiers. The organic modifiers with
different functional groups play an important role in deciding the size and structure of the nickel
nanoparticles. As compared to the conventional RANEYs Ni catalyst, all the as-prepared nickel
nanoparticles showed high catalytic activity and selectivity in the hydrogenation of p-nitrophenol
to p-aminophenol. The catalytic activities of the as-prepared nickel nanoparticles generally
increased with decreasing the particle size. The crystal structure of nickel nanoparticles also had
an important impact on the catalytic activity.
because they are easily oxidized in water,13,29 which limits
Introduction
their application. In order to avoid the oxidation of nickel
nanoparticles, organic solvents, such as tetrahydrofuran,14
Conventionally, p-aminophenol, as an important chemical
hexadecylamine,20 ethylene glycol,21 and ethanol,22 were used
intermediate in the preparation of analgesic and antipyretic
drugs,1 dyestuffs,2 and photographic developers,3 is produced
in the synthesis of phase pure nickel nanoparticles by a wet
by iron-acid reduction of p-nitrophenol. In this process, the
generation of a large amount of Fe–FeO sludge (1.2 kg kgÀ1
In this paper, we report the synthesis of phase pure nickel
chemical method.
product) causes a serious pollution problem.4 Now the
nanoparticles via a simple chemical reduction route in anhydrous
production of p-aminophenol is mainly via a single step
ethanol. The effects of organic modifiers with different functional
catalytic hydrogenation of nitrobenzene using both noble
groups, such as poly(ethylene glycol) 200, poly(ethylene glycol)
metal (Pt) and strong mineral acid (H2SO4) as co-catalysts.4,5
6000, Tween 40, Tween 80, and sodium dodecyl sulfonate, on
The disadvantages of this process are that strong corrosive
the size and crystal structure of the resultant nickel nano
sulfuric acid and high cost noble metal are used and that
particles were investigated. The catalytic performance of
aniline as a competitive byproduct is largely and unavoidably
nickel nanoparticles in the hydrogenation of p-nitrophenol
formed. For these reasons, from both economic and environ-
to p-aminophenol was studied and compared with that of a
conventional RANEYs Ni catalyst.
mental points of view, there has recently been increasing
interest in the direct hydrogenation of p-nitrophenol to
p-aminophenol catalyzed by nickel,6,7 especially by nickel
nanoparticles.8–10 However, the catalytic performance of nickel
Experimental
nanoparticles in the hydrogenation of p-nitrophenol to
Materials
p-aminophenol was seldom investigated until now. In parti-
cular, the study of the effect of size and crystal structure of
Nickel oxalate (NiC2O4Á2H2O, 98.5%), hydrazine hydrate
nickel nanoparticles on their catalytic activities is lacking.
(N2H4ÁH2O, 85%), poly(ethylene glycol) (PEG, MW = 200
It is well known that organic modifiers are used in the size-
controlled synthesis of nanosized metals. In the size-controlled
synthesis of nickel nanoparticles, several organic modifiers,
such as sodium carboxyl methyl cellulose,11 tetrabutylammo-
nium bromide,12,13 sulfonated polybutadiene,14 and cetyltri-
methylammonium bromide,15 have been used to prepare
nickel nanoparticles by wet chemical reduction in aqueous
solution16–19 or in organic media,14,20–25 microemulsion,26,27
and hydrothermal15,28 techniques. Among these techniques,
wet chemical reduction is a facile method. But it is difficult to
prepare phase pure nickel nanoparticles in aqueous media
or 6000), Tween 40, Tween 80, sodium dodecyl sulfonate,
sodium hydroxide (96%), anhydrous ethanol (99.8%), and
p-nitrophenol were purchased from the Shanghai Chemical
Reagent Co. Ltd. All chemicals were used as received without
further purification. Anhydrous ethanol was used as a solvent
throughout all the experiments.
Preparation of nickel nanoparticles
Nickel nanoparticles were prepared by reducing nickel oxalate
with hydrazine hydrate in the presence of organic modifiers
with different functional groups, such as PEG 200, PEG 6000,
Tween 40, Tween 80, and sodium dodecyl sulfonate. Typically,
the organic modifier (0.37 g, 0.01 M) and nickel oxalate
(2.49 g, 0.1 M) were dissolved in the anhydrous ethanol
solution (70 mL) by ultrasonic dispersion for 30 min. While
Faculty of Chemistry and Chemical Engineering, Jiangsu University,
Xuefu Road 301, Zhenjiang, 212013, China. E-mail: yin@ujs.edu.cn;
Fax: +86-511-88791800; Tel: +86-511-88787591
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the mixture was heated to 60 1C, an ethanol solution of NaOH
(1.0 M, 20 mL) was added dropwise into it to adjust the pH of
the reaction solution to 12. After that, hydrazine hydrate
ethanol solution (7.5 mL in 10 mL anhydrous ethanol) was
added dropwise into the mixture and heated up to 80 1C for
8 h under stirring. The resultant nickel nanoparticles were
cooled to room temperature and kept in anhydrous ethanol
solution. Nickel nanoparticles were centrifuged and washed
with anhydrous ethanol when they were used as the catalysts
in the hydrogenation of p-nitrophenol.
chromatography (HPLC, Varian ProStar 210) equipped with
an ultraviolet detector set at a wavelength of 254 nm and a C18
column (5 mm, 4.6 mm  250 mm). A mixture of methanol and
acetic acid (0.2 wt%) aqueous solution (3 : 2 v/v) was used as a
mobile-phase at a flow rate of 1 mL minÀ1
.
Results and discussion
XRD analysis
Fig. 1 shows the XRD patterns of the samples prepared by
using organic modifiers with different functional groups. The
diffraction peaks appearing at 2y = 44.5, 51.9, and 76.51 were
indexed as the (111), (200), and (220) planes of the face-
centered-cubic (fcc) nickel (JCPDS No. 04-0850), respectively.
No impurity diffraction peaks, such as nickel oxides or nickel
hydroxides, were detected, indicating that phase pure metal
nickel was prepared under the present experimental condi-
tions. The crystallite sizes of the Ni (111) in the samples
calculated by using Scherrer’s equation were 16.9 nm,
13.1 nm, 19.7 nm, 11.8 nm, and 8.5 nm, respectively, when
the samples were prepared by using PEG 200, PEG 6000,
Tween 40, Tween 80, and sodium dodecyl sulfonate as organic
modifiers (Table 1).
Preparation of RANEYs Ni catalyst
RANEYs Ni catalyst was prepared by the following proce-
dure: 1.0 g of Ni–Al alloy with a weight ratio of 47 : 53 and
particle sizes in a range of 38–45 mm was added to an aqueous
solution of NaOH (6.0 M, 10 mL) under gentle stirring at
50 1C. After that, the reaction solution was heated to 80 1C
and stirred for 1 h. The black precipitate was washed with
distilled water to neutrality and then washed with anhydrous
ethanol to replace water. The as-prepared RANEYs Ni
catalyst was kept in anhydrous ethanol.
Catalyst characterization
The as-prepared nickel nanoparticles were characterized by
X-ray diffraction (XRD) on a Rigaku, D-max 2200 X-ray
diffractometer with Cu-Ka radiation at l = 1.5406 A. The
crystallite sizes of the Ni (111) in the samples were calculated
by using Scherrer’s equation: D = Kl/(Bcosy), where K was
taken as 0.89, and B was the full width of the diffraction line at
half of the maximum intensity. High resolution transmission
electron microscopy (HRTEM) images and the corresponding
selected area electron diffraction (SAED) patterns were obtained
on a microscope (JEM-2100) operated at an acceleration voltage
of 200 kV to characterize the morphologies and the crystal
structures of the as-prepared nickel nanoparticles. The TEM
specimens were prepared by placing a drop of nickel nano-
particle colloids onto a copper grid coated with a layer of
amorphous carbon and evaporating the solvent in air at room
temperature. The average particle sizes of the nickel nano-
particles were measured from the TEM images by counting at
least 150 individual particles. The average particle sizes of the
samples were calculated by a weighted average method according
to the individual particle sizes of the all counted particles.
Fourier transform infrared (FTIR) spectra were collected on a
Fourier transform infrared spectrometer (Nicolet Nexus 470)
by KBr pellet technique.
Catalytic hydrogenation of p-nitrophenol to p-aminophenol
The hydrogenation of p-nitrophenol was carried out in a
1000 mL capacity stainless steel autoclave fitted with a
magnetically driven impeller. The autoclave was charged with
3.0 g of p-nitrophenol dissolved in 150 mL anhydrous ethanol
and 0.09 g of as-prepared nickel nanoparticles or RANEYs
Ni catalyst. Firstly, the autoclave was purged with nitrogen for
10 min. Then hydrogen from a cylinder was introduced and
raised to 0.8 MPa. Under stirring at 600 rpm, the reaction was
carried out at a temperature of 100 1C for different times. The
liquid phase sample was analyzed by high-performance liquid
Fig. 1 XRD patterns of nickel nanoparticles prepared by reducing
nickel oxalate with hydrazine hydrate and using (a) PEG 200, (b) PEG
6000, (c) Tween 40, (d) Tween 80, and (e) sodium dodecyl sulfonate as
organic modifiers, respectively.
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Table 1 Crystallite size of Ni (111) (XRD) and average particle size
(TEM) of the nickel nanoparticles prepared by using different organic
modifiers, and conversion of p-nitrophenol at 2 h
Crystallite
size/nm
Average particle
size/nm
Conversion
(%)
Organic modifiers
Tween 40
PEG 200
PEG 6000
Tween 80
Sodium dodecyl
sulfonate
19.7
16.9
13.1
11.8
8.5
297 Æ 154.5
284 Æ 100.5
184 Æ 24.6
178 Æ 19.7
60 Æ 9.3
14.4
45.5
52.9
62.6
80.1
TEM analysis
Fig. 2 shows the TEM and HRTEM images of the nickel
nanoparticles prepared by reducing nickel oxalate with
hydrazine hydrate in the presence of organic modifiers with
different functional groups. Fig. 3 shows the particle size
distributions of the as-prepared nickel nanoparticles. The data
of the average particle sizes are listed in Table 1.
Fig. 2a shows the TEM image of the nickel nanoparticles
prepared by using PEG 200 as an organic modifier. The
resultant nickel nanoparticles were sphere-like in shape with
an average diameter of ca. 284 nm and a size distribution
ranging from 67 nm to 733 nm (Fig. 3). The inset shows the
SAED pattern obtained by directing the incident electron
beam perpendicular to one of the nanoparticles. The diffraction
rings are indexed to fcc nickel metal and indicate that the
nickel nanoparticles are of a polycrystalline structure. Fig. 2b
shows the HRTEM image of the nickel nanoparticles and the
inset shows the corresponding inverse fast Fourier transform
(FFT) image. It was found that point defects were present in
the resultant nickel nanoparticles. Fig. 2c shows the TEM
image of nickel nanoparticles prepared by using PEG 6000 as
an organic modifier. It was found that the resultant nickel
nanoparticles had an average diameter of ca. 184 nm and a size
distribution ranging from 78 nm to 275 nm (Fig. 3). The inset
in Fig. 2c shows the corresponding SAED pattern, indicating
that the nickel nanoparticles were of a polycrystalline structure.
The HRTEM (Fig. 2d) and FFT images (inset in Fig. 2d, top)
show that the resultant polycrystalline nickel nanoparticles
had a twin crystal structure. At the same time, the corresponding
inverse FFT image (inset in Fig. 2d, bottom) indicates that
point defects were also present in the resultant nickel nano-
particles.
Fig. 2 TEM and HRTEM images of the nickel nanoparticles pre-
pared by reducing nickel oxalate with hydrazine hydrate and using
(a, b) PEG 200, (c, d) PEG 6000, (e, f) Tween 40, (g, h) Tween 80, and
(i, j) sodium dodecyl sulfonate as organic modifiers, respectively. The
insets in the TEM images show the corresponding SAED patterns and
the diffraction rings are assigned to {111}, {200}, {220}, and {311}
reflections of fcc nickel metal, respectively. The insets in the HRTEM
images show the corresponding FFT and inverse FFT images.
Fig. 2e shows the TEM image of the nickel nanoparticles
prepared in the presence of Tween 40 as an organic modifier. It
was found that the spherical nickel nanoparticles with an
average diameter of ca. 297 nm and a size distribution ranging
from 50 nm to 1000 nm were prepared (Fig. 3). The SAED
pattern (inset in Fig. 2e) and the HRTEM image (Fig. 2f)
indicate that nickel nanoparticles with a polycrystalline structure
were prepared. The inverse FFT image (inset in Fig. 2f, bottom)
does not show any defects, indicating that the resultant nickel
nanoparticles had a comparatively perfect internal structure.
Fig. 2g shows the TEM image of the nickel nanoparticles
prepared in the presence of Tween 80 as an organic modifier.
The resultant spherical nickel nanoparticles had an average
diameter of ca. 178 nm and a size distribution ranging from
95 nm to 238 nm (Fig. 3). The corresponding SAED pattern
(inset in Fig. 2g) indicates that fcc nickel nanoparticles with a
polycrystalline structure were formed. The HRTEM and the
inset inverse FFT images (Fig. 2h) show that dislocations were
present in the resultant nickel nanoparticles.
Fig. 2i shows the TEM image of the nickel nanoparticles
prepared in the presence of sodium dodecyl sulfonate as an
organic modifier. The resultant spherical nickel nanoparticles
had an average diameter of ca. 60 nm and a size distribution
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Fig. 3 Particle size distributions of the Ni samples prepared by using
PEG 200, PEG 6000, Tween 40, Tween 80, and sodium dodecyl
sulfonate as organic modifiers, respectively. D, average particle size.
ranging from 18 nm to 127 nm (Fig. 3). The SAED pattern
(inset in Fig. 2i) indicates that fcc nickel nanoparticles with
polycrystalline structure were formed. The HRTEM and the
inverse FFT image (Fig. 2j) show that dislocations were
present in the resultant nickel nanoparticles.
It was found that the average particle sizes of the as-
prepared nickel nanoparticles calculated from TEM images
are larger than those calculated by using Scherrer’s equation.
It is reasonable to conclude that all the as-prepared nickel
nanoparticles are of polycrystalline structures, consistent with
that obtained by SAED analysis.
FTIR analysis
In order to investigate the interaction between the organic
modifiers and the nickel nanoparticles, FTIR spectra of the
samples were measured. The representative FTIR spectra of
the pure organic modifiers and the resultant nickel nano-
particles are shown in Fig. 4.
Fig. 4 FTIR spectra of pure (a) PEG 6000, (c) Tween 80, (e) sodium
dodecyl sulfonate, and unwashed nickel nanoparticles prepared by
reducing nickel oxalate with hydrazine hydrate and using (b) PEG
6000, (d) Tween 80, and (f) sodium dodecyl sulfonate as organic
modifiers, respectively.
Fig. 4a and b show the FTIR spectra of pure PEG 6000 and
unwashed nickel nanoparticles prepared in the presence of
PEG 6000 as an organic modifier. The FTIR spectra of pure
PEG 6000 show that the C–O–C stretching vibrations appear
observed in the FTIR spectra of pure sodium dodecyl sulfo-
nate (Fig. 4e).34 The appearance of the band at nmax/cmÀ1
1049 (n(–SO3À)) in the unwashed sample (Fig. 4f) revealed that
there was an interaction between the sulfonic group of sodium
dodecyl sulfonate and the nickel nanoparticles.
at nmax/cmÀ1 1106 (n(C–O–C)trans) and 1148 (n(C–O–C)gauche
)
(Fig. 4a).30,31 The appearance of the band at nmax/cmÀ1 1062
(n(C–O–C)) in the FTIR spectra of the unwashed sample
(Fig. 4b) indicated that the ether bond of PEG 6000 molecules
interacted with the surface of the nickel nanoparticles.
When the as-prepared nickel nanoparticles were washed
with anhydrous ethanol, no IR bands of the organic modifiers
were observed in the FTIR spectra, meaning that the organic
modifiers were completely removed by washing with ethanol.
The interaction between organic modifier and nickel nano-
particle affected the particle size and structure.
Fig. 4c and d show the FTIR spectra of pure Tween 80 and
unwashed nickel nanoparticles. In the FTIR spectra of pure
Tween 80 (Fig. 4c), the bands appearing at nmax/cmÀ1 1736
and 1109 are assigned to the stretching vibrations of CQO and
C–O–C, respectively.32,33 These bands were shifted to nmax/cmÀ1
1571 (CQO) and 1058 (C–O–C) in the FTIR spectra of the
unwashed sample (Fig. 4d), indicating that Tween 80 was
adsorbed on the surface of the nickel nanoparticles.
Catalytic activity
The catalytic activities of the resultant nickel nanoparticles are
illustrated in Fig. 5. The selectivity of p-aminophenol was
100% while the hydrogenation of p-nitrophenol was catalyzed
by all the resultant nickel nanoparticles. After the first 2 h, the
initial p-nitrophenol hydrogenation rates of the nickel
Fig. 4e and f show the FTIR spectra of pure sodium dodecyl
sulfonate and unwashed nickel nanoparticles. The bands
appearing at nmax/cmÀ1 1065 and 1056 (n(–SO3À)) were
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catalyzed by RANEYs Ni catalyst. The byproduct was
ascribed to the product of benzene ring hydrogenation, which
was caused by the micropores of RANEYs Ni.8,35 The results
showed that the nickel nanoparticles had higher catalytic
activity and selectivity than RANEYs Ni catalyst in the
hydrogenation of p-nitrophenol to p-aminophenol.
Conclusions
In summary, nickel nanoparticles with different sizes and
different structures were prepared by reducing nickel oxalate
with hydrazine hydrate in the presence of organic modifiers,
such as PEG 200, PEG 6000, Tween 40, Tween 80, and sodium
dodecyl sulfonate, respectively. The ability of the organic
modifiers on decreasing particle size of nickel nanoparticles
is in an order of sodium dodecyl sulfonate (60 nm) 4 Tween
80 (178 nm) 4 PEG 6000 (184 nm) 4 PEG 200 (284 nm) 4
Tween 40 (297 nm). PEG 6000 caused the formation of nickel
nanoparticles with a twin crystal structure and point defects
while PEG 200 resulted in the formation of nickel nanoparticles
with point defects. When Tween 80 was used as an organic
modifier, the nickel nanoparticles with dislocations were formed.
The nickel nanoparticles with a relatively perfect internal struc-
ture were prepared in the presence of Tween 40 as an organic
modifier. When sodium dodecyl sulfonate was used as an
organic modifier, nickel nanoparticles with dislocations were
formed. The catalytic activity of the nickel nanoparticles in the
hydrogenation of p-nitrophenol to p-aminophenol generally
increased with decreasing their sizes. However, the catalytic
activity of nickel nanoparticles depended not only on the size
but also on the crystal structure. All the resultant nickel
nanoparticles showed higher catalytic activity and selectivity
than the RANEYs Ni catalyst.
Fig. 5 Catalytic activities of the nickel nanoparticles prepared by
reducing nickel oxalate with hydrazine hydrate in the presence of (’)
PEG 200, (K) PEG 6000, (m) Tween 40, (.) Tween 80, and (E)
sodium dodecyl sulfonate as organic modifiers, respectively, and the
(&) RANEYs Ni catalyst in the hydrogenation of p-nitrophenol to
p-aminophenol.
nanoparticles were 5.63, 4.40, 3.72, 3.20, and 1.01 molnitrophenol
/
molcatÁh, and the p-nitrophenol conversions were 80.1%,
62.6%, 52.9%, 45.5%, and 14.4% (Table 1) while the nickel
nanoparticles were prepared by using sodium dodecyl sulfonate,
Tween 80, PEG 6000, PEG 200, and Tween 40 as organic
modifiers, respectively. After reaction for 8 h, the conversions
of p-nitrophenol were up to 97%, 97%, 98%, 97%, and 80%,
respectively. By comparing the initial p-nitrophenol hydro-
genation rates at the first 2 h, it was found that the catalytic
activities of the nickel nanoparticles were in an order of
sodium dodecyl sulfonate (60 nm) 4 Tween 80 (178 nm) 4
PEG 6000 (184 nm) 4 PEG 200 (284 nm) c Tween 40
(297 nm). The catalytic activity of the nickel nanoparticles
generally increased with decreasing their sizes.
Acknowledgements
The authors would like to thank Professor K. Chen (Jiangsu
University) for supporting TEM and HRTEM measurements
of nickel samples. This work was financially supported by
research funds from Jiangsu Province Education Bureau
(1221310008) and Zhenjiang Science and Technology Bureau
(GJ2006006) and Jiangsu University (1281310001).
Interestingly, although the nickel nanoparticles prepared by
using PEG 200 and Tween 40 as organic modifiers had
comparable average particle sizes, the catalytic activity of
the former was ca. 3.2 times higher than that of the later. As
certified by HRTEM analysis, the nickel nanoparticles prepared
by using PEG 200 as an organic modifier had point defects.
However, when Tween 40 was used as an organic modifier, the
as-prepared nickel nanoparticles had a comparatively perfect
internal structure. It can be concluded that the catalytic
activity of the nickel nanoparticles depends on the synergistic
effect of the crystal structure and the size.
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