Ni/MWCNT-supported palladium nanoparticles as magnetic catalysts for selective oxidation of benzyl alcohol

Article abstract of DOI:10.1071/CH12484

Well dispersed Pd@Ni bimetallic nanoparticles on multi-walled carbon nanotubes (Pd@Ni/MWCNT) are prepared and used as catalysts for the oxidation of benzyl alcohol. Scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray sp

Full text of DOI:10.1071/CH12484

CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH12484
Ni/MWCNT-Supported Palladium Nanoparticles
as Magnetic Catalysts for Selective Oxidation
of Benzyl Alcohol
A
A
A
A
A
Mingmei Zhang, Qian Sun, Zaoxue Yan, Junjie Jing, Wei Wei,
A
A B
,
A
Deli Jiang, Jimin Xie,
and Min Chen
A
School of Chemistry and Chemical Engineering, Jiangsu University,
Zhenjiang 212013, China.
B
Corresponding author. Email: Xiejm391@sohu.com
Well dispersed Pd@Ni bimetallic nanoparticles on multi-walled carbon nanotubes (Pd@Ni/MWCNT) are prepared and
used as catalysts for the oxidation of benzyl alcohol. Scanning electron microscopy, transmission electron microscopy,
energy-dispersive X-ray spectroscopy analysis, and X-ray diffraction were performed to characterise the synthesised
catalyst. The results show a uniform dispersion of Pd@Ni nanoparticles on MWCNT with an average particle size of
4.0 nm. The as synthesised catalyst was applied to the oxidation of benzyl alcohol. A 99 % conversion of benzyl alcohol
and a 98 % selectivity of benzaldehyde were achieved by using the Pd@Ni/MWCNT (Pd: 0.2 mmol) catalyst with water as
a solvent and H2O2 as oxidant at 808C. The catalytic activity of Pd@Ni/MWCNT towards benzyl alcohol is higher than
that of a Pd/MWCNT catalyst at the same Pd loadings. The catalyst can be easily separated due to its magnetic properties.
Manuscript received: 23 October 2012.
Manuscript accepted: 11 January 2013.
Published online: 20 February 2013.
Introduction
showed excellent selectivity towards benzaldehyde in selective
oxidations of benzyl alcohol in the absence of solvent and
without a base.[10] Pd as one of noble metals occurs in nature
at very low levels of abundance although as a catalysts in various
reactions it is highly effective. Hybrid alloy structures of Pd
(a noble metal) and Ni (a non-noble metal) not only combines
the properties of the individual constituents but also shows an
enhancement in specific properties because of the synergistic
effects and the rich diversity of the compositions.[18–28] Adzic
et al. demonstrated the high activity and long-term stability of a
Pt monolayer deposited on a metal, a metal alloy, or core–shell
nanoparticles whose activity can surpass that of the state-of-the-
art carbon-supported all-Pt electrocatalysts.[24]
In this paper, multiwalled carbon nanotube (MWCNT)-
supported Pd@Ni bimetallic nanocatalysts were synthesised
by two-steps and their catalytic efficiency for the selective
oxidation of benzyl alcohol were studied. The mechanisms
toward Pd@Ni/MWCNT, i.e. the formation of Ni-nanoparticle-
loaded MWCNT and Pd-salt-loaded Ni/MWCNT, were
investigated. The hydrogen peroxide oxidation of benzyl alco-
hol catalyzed by Pd@Ni/MWCNT nanoparticles was conducted
in water for environmental compatibility. The interactions
between Ni nanoparticles and Pd nanoparticles, such as crystal-
line nickel entering into the palladium crystal lattice leading to a
synergistic effect, a d-band centre shift,[27] and a Pd skin effect,
are favourable for the promotion of the catalytic activity and
stability of Pd@Ni/MWCNT. Several experimental parameters,
such as amount of catalyst, temperature, reaction time, and the
catalytic activity of recycling were systematically studied to
evaluate the catalytic performances of Pd@Ni/MWCNT
Selective oxidation of benzyl alcohol on noble-metal-based
catalysts has been widely studied during the past decades.[1–6]
Other than hydrogen peroxides, gas phase oxidation at an ele-
vated temperature by air/O2 also provides an alternative
route.[3,4] Palladium (Pd) as a most versatile catalyst ingredient
has been widely used in organic reactions.[5,6] As a promising
way to achieve larger catalyst utilisation, supports were adopted
to disperse the Pd nanoparticles with a smaller size, which
greatly improved the ratio of use of noble metal to catalytic
activity. Carbon materials with various shapes as Pd catalyst
supports have been intensively investigated due to their low
weight, high specific area, and chemical inertia. Yan et al.
synthesised three kinds of hollow carbon hemispheres, and
loaded Pd nanoparticles using an intermittent microwave heat-
ing method, to generate electrocatalysts for alcohol electro-
oxidation in fuel cells. They found a higher catalytic activity
than using the Vulcan XC-72 carbon powder as support.[7,8]
Carbon nanotubes (CNT), as novel carbon materials with
excellent chemical inertia, are promising supports for catalysts.
Xiang et al. has reported that CNT-supported Pd catalysts
(Pd/CNT) are more selective for the hydrogenation of aceto-
phenone to give a-phenylethanol than the commercially avail-
able, activated carbon-supported Pd catalyst.[9]
Another effective method to enhance the activity of catalysts
is to mix other elements with the precious metal to generate
intermetallic compounds and alloys, which has been proven to
be an effective method in making better use of the precious
metal.[10–17] Deplanche et al. synthesised bimetallic Au/Pd
nanoparticles supported on bacterial cells and the catalyst
Journal compilation Ó CSIRO 2013
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B
M. Zhang et al.
nanoparticles in the selective oxidation of benzyl alcohol.
Pd@Ni/MWCNT exhibited a much higher catalytic activity
compared with an equal amount of palladium nanoparticles in
the selective oxidation of benzyl alcohol.
evaporated on a rotary evaporator, and then analysed by gas
chromatography. The recycled organic layer was analysed by a
7890A gas chromatograph (Agilent Technology Inc., San Jose,
CA, USA) equipped with a flame-ionisation detector. The GC
yield was obtained from the normalisation method.
Experimental
Measurements
Preparation of Acid-Treated MWCNT
The morphology of the as-prepared samples were examined by
scanning electron microscopy (SEM) using a field emission
instrument (Hitachi S-4800 II, Japan) and transmission electron
microscopy (TEM) (JEOL-JEM-2010, Japan) operating at
120 kV. The phase purity and crystal structure of the obtained
samples were examined by X-ray diffraction (XRD) using a D8
Advance X-ray diffractor (Bruker AXS Company, Germany)
MWCNT used in this work were purchased from Shenzhen
Nanotechnologies Port Co. Ltd (Shenzhen, China) with a
diameter of 20–40 nm, length of 6–15 mm, and purity of
98 wt.-%. The MWCNT were purified by refluxing in concen-
trated nitric acid at 1008C for 12 h, and then washed and filtered
with deionised water.[29,30] In order to generate significant
amounts of functional groups on the surface of the MWCNT,
a typical treatment was as follows:[31] 50 g of MWCNT, 60 mL
of H2SO4, and 20 mL of HNO3 were mixed with sonication for
10 min, and then placed in an oil bath at 808C with vigorous
stirring for 1 h. The resulting product was filtered off and
washed with deionised water five times, and dried in a vacuum
oven at 608C for 12 h.
˚
equipped with CuKa radiation (l 1.5406 A), employing a scan-
ning rate of 0.02 deg sꢀ1 in the 2y range from 108 to 808. The
metal contents of the catalysts were analysed by inductively
coupled plasma spectroscopy (ICP, Optima2000DV, USA). The
recycled organic layers were analysed by a 7890A gas chro-
matograph (Agilent Technology Inc., USA) equipped with a
flame-ionisation detector and HP-5 capillary column (30 m ꢁ
0.32 mm ꢁ 0.25 mm), N2 was the carrier gas, and the following
settings were used: column temperature: 1208C, oven temper-
ature: 2308C, temperature of boil room: 2508C. The flow rate of
Preparation of the Ni/MWCNT Catalyst
To synthesise the Ni/MWCNT compounds, 4 g of acid-treated
MWCNT was dispersed in 100 mL of ethylene glycol (EG) by
sonication for 60 min. In this reaction, EG acts as a reducing,
stabilising, and dispersing agent. Nickel acetate (5 g) was then
added to the solution under vigorous stirring, and then subjected
to microwave heating in a microwave oven at a temperature of
1808C operated at 750 W. The pH of the entire solution was
adjusted to 10 by adding NaOH (2.0 M). At the end of the
heating, the solution was cooled to room temperature, and the
product was isolated by several washes with distilled water to
remove the excess EG and subsequent separation by sintered
discs. Finally, the nickel-impregnated MWCNT (Ni/MWCNT)
were obtained. The Ni contents were analysed by inductively
coupled plasma spectroscopy (ICP, Optima2000DV, USA)
analysis, which showed 30.2 wt-% of Ni in the Ni/MWCNT
compound.
hydrogen, air, and nitrogen were 40, 300, and 30 mL minꢀ1
.
Results and Discussion
The Formation Mechanism of Pd@Ni/MWCNT
The schematic illustration for the formation of Pd@Ni/
MWCNT nanocomposites is shown in Fig. 1. Once MWCNT
was functionalised, it was dispersed thoroughly in EG by soni-
cation for 10 min. Nickel acetate was then added to the solution
under vigorously stirring, and the mixture was subjected to
microwave heating in a microwave oven. Finally the compound
was isolated by several washes with distilled water and mono-
crystalline nickel nanoparticles supported on MWCNT
(Ni/MWCNT) were obtained. After the Ni/MWCNT was added
to a H2PdCl4 solution and continuously stirred for 14 h, well
dispersed Pd@Ni nanoparticles on MWCNT were obtained.
Preparation of the Pd@Ni/MWCNT Catalyst
Pd@Ni/MWCNT catalysts were synthesised by a replacement
method. Typically, 20 mL of 0.372 M H2PdCl4 and 2 g of
Ni/MWCNT were dispersed in 100 mL of distilled water. The
resulting solution was uniformly dispersed by sonification for
10 min, and then vigorously stirred for 14 h at room temperature.
The black solid was separated using sintered discs, washed with
deionised water five times, and finally dried in a vacuum oven at
608C. For comparison, Pd nanoparticles supported on MWCNT
(Pd/MWCNT) were also obtained directly by reducing H2PdCl4
in a MWCNT suspension using formic acid as the reducing
agent. The theoretical Pd contents in both Pd@Ni/MWCNT and
Pd/MWCNT were targeted at 20 wt-%. ICP analysis gave the
actual Pd contents as 19.5 wt-% for Pd@Ni/MWCNT and
19.2 wt-% for Pd/MWCNT.
FTIR Spectra Analysis
In order to identify the chemical groups on the surface of the
MWCNT, we conducted FTIR experiments and the results are
shown in Fig. 2. The broad intense band around 3440 cmꢀ1 in
Fig. 2a can be attributed to the stretching vibrational mode of
O–H groups. This band might have resulted due to –OH func-
tional groups forming during the purification process. The peak
at 1638 cmꢀ1 is assigned to the C¼C stretching of the MWCNT.
A small peak at around 1385 cmꢀ1 is due to O–H bending
deformation in –COOH. For the HNO3-acid treated MWCNT
sample (HNO3-f-MWCNT, Fig. 2b), the peaks are obviously
weak compared to the H2SO4/HNO3-treated MWCNT (H2SO4/
HNO3-f-MWCNT, Fig. 2a). Almost no peak can be seen in the
non-acid-treated MWCNT (Fig. 2c), which is further evidence
that functional groups are generated on the surface of MWCNT
by H2SO4/HNO3 acid treatment.
Procedure for Alcohol Catalytic Oxidation
A conical flask was filled with benzyl alcohol (1.0 mmol),
K2CO3 (3.0 mmol), Pd@Ni/MWCNT, or Pd/MWCNT (Pd:
0.2 mmol), and H2O (20.0 mL). The mixture was stirred and
dispersed for 1 h before adding freshly prepared H2O2 at a given
temperature. Upon completion of the reaction the organic
products were extracted from the reaction mixture with
dichloromethane (20 mL). The extracted organic layer was
Morphology and Element Analysis
The morphology and particle size of acid-treated MWCNT,
Ni/MWCNT, Pd@Ni/MWCNT, and Pd/MWCNT were exam-
ined by FESEM and TEM as shown in Fig. 3. Fig. 3a shows a
typical SEM image of acid-treated MWCNT. From Fig. 3band 3d

Pd@Ni/MWCNT as a Catalyst
C
Ni(NO3)2
Microwave method
Stirring 2 days
Acid-functionalised MWCNT
Pd
Ni
Pd@Ni/MWCNT
Fig. 1. Schematic diagram of the synthesis of Pd@Ni nanocatalysts on multiwalled carbon nanotubes (MWCNT).
be inferred that the Ni particles were supported on the MWCNT
first, and then the Pd particles coated the Ni nanoparticles. At the
same time Pd particles were fixed to the MWCNT because the
bottom of the Pd also contacted with the MWCNT. Possibly,
both palladium and nickel nanoparticles have strong interactions
with the MWCNT surface, which restrains the aggregation and
the growth of the nanoparticles to form larger particles, resulting
in the uniform distribution of these nanoparticles. The particle
size distribution derived from the TEM results (the insets in
Fig. 4a, b) further illustrate this. The elemental analysis by EDS
(Fig. 4a) proved that the Ni/MWCNT are composed of C and Ni.
It is seen that Pd@Ni nanoparticles supported on pristine
MWCNT (Fig. 5a) show an uneven distribution and aggregation
in comparison with Pd@Ni nanoparticles supported on acid-
treated MWCNT (Fig. 5b). Thus, it can provide us with
straightforward information that acid-treated MWCNT contain
abundant oxygen functional groups on the surface, which can
efficiently disperse metal nanoparticles and result in a higher
catalytic activity.
(c)
(b)
(a)
3500
3000
2500
2000
1500
1000
Wavenumbers [cmϪ1
]
Fig. 2. FTIR spectra of H2SO4/HNO3-treated multiwalled carbon nano-
tubes (a) (MWCNT), (b) HNO3-treated MWCNT, and (c) untreated
MWCNT.
XRD Analysis
The crystal structure and the phase purity of MWCNT,
Ni/MWCNT, Pd/MWCNT, and Pd@Ni/MWCNT particles
were determined by XRD as shown in Fig. 6a. The broad peaks
at 2y ¼ 26.078 are associated with C (002) planes of the graphite-
like structure of the MWCNT. The other five peaks of the
Pd@Ni/MWCNT are characteristic of face centred cubic (fcc)
crystalline Pd and Ni, corresponding to the planes of Ni (111),
(200), and (220) at 2y values of ,44.818, 51.998, and 76.628, and
Pd (111), (200), and (220) at 2y values of ,40.268, 46.818, and
68.278. Fig. 6a reveals an obvious shift in the position of the Ni
(200) peak in the Pd@Ni/MWCNT compared with that in the
Ni/MWCNT (Fig. 6a, curve b). The shift of the diffraction peak
reveals that there is strong interaction between partial Ni and
Pd.[32,33]
fraction peak of Ni in Fig. 6a, curve d, which further suggests
that Ni atoms form a core with Pd atoms partly forming an
outside layer. As displayed in Fig. 6b, a positive shift of the Pd
peaks occur for Pd@Ni/MWCNT compared with the
Pd/MWCNT, indicating that the Ni atoms enter into the Pd
crystals which causes the narrow transformation of the Pd
crystal lattice distance. During the aerobic selective oxidation of
an alcohol it has been demonstrated that the Pd (111) plane is the
we can see that the dispersion of Pd@Ni nanoparticles on acid-
treated MWCNT is very uniform. Moreover, Ni nanoparticles
supported on acid-treated MWCNT with a diameter ranging
from 6 to 10 nm can be observed (Fig. 3c), evidencing that EG
successfully made the Ni impregnated in MWCNT transform
into monocrystalline nickel nanoparticles at 1808C under
microwave heating in a microwave oven. The elemental anal-
ysis by EDS proved that the Ni/MWCNT are composed of C and
Ni. TEM was further used to explore the morphology of Pd@Ni
nanoparticles on MWCNT composites (Fig. 3d). It shows a
large number of extremely small nanoparticles distributed
homogeneously on the external surface of the acid-treated
MWCNT. Furthermore, the Pd@Ni nanoparticles on the
MWCNT have a narrow size range from 3 to 6 nm with regular
shapes. The high-resolution TEM image describing the crys-
talline nature of the Pd@Ni nanoparticles is shown in Fig. 3f.
The single crystalline Pd@Ni particles have lattice planes with
an interlayer distance of 0.203 nm, which are indexed to
Ni (111) crystal planes. The outer layer of the nanoparticles
show a different contrast, with the parallel lattice fringes mea-
sured as 0.224 nm, which corresponds to Pd (111) planes. It can
The peaks of Pd are obviously stronger than the dif-

D
M. Zhang et al.
(a)
(b)
100 nm
100 nm
(c)
(d)
20 nm
(e)
20 nm
(f)
2 nm
10 nm
Fig. 3. Field effect scanning electron microscopy images of (a) acid-treated multiwalled carbon nanotubes (MWCNT) and
(b) Pd@Ni/MWCNT. Transmission electron microscopy (TEM) images of (c) Ni/MWCNT and (d) Pd@Ni/MWCNT. Magnified
TEM image of Pd@Ni/MWCNT (the inset is the corresponding SAED pattern) (e). (f) High-resolution TEM image of
Pd@Ni/MWCNT.
catalytic centre and most alcohols and O2 are adsorbed and
desorbed on it.[34] The relatively strong diffraction peak of the
Pd (111) plane in Fig. 6a, curve d, reveals that the as-made
catalyst has catalytically active centres and can promote the
selective oxidation of alcohols. Based on the Scherer’s equation
shown below (Eqn 1) and the full width half-maximum peak of
the Pd (111) crystal plane, the average crystalline sizes of all
these samples were estimated according to the line width anal-
ysis of the Pd (111) diffraction peak. As can be seen, the crystal
mean size is found to be 4.0 nm for Pd@Ni/MWCNT
where D is the average particle size, l ¼ 0.15406 nm (XRD
wavelength corresponding to CuKa radiation), b is the width
(radians) of the peak at half height, and y is the diffraction angle.
Pd@Ni/MWCNT as Catalysts for Selective Oxidation
of Benzyl Alcohol
The catalytic activity of Pd@Ni/MWCNT nanoparticles for the
oxidation of benzyl alcohol is systematically examined. Clean
catalytic syntheses in water under mild conditions are highly
desirable processes from an economic to environmental point of
view. Therefore, water as the reaction media and hydrogen
peroxide (H2O2) as oxygen resources are chosen. Considering
their high activity and low cost, Pd@Ni nanoparticles are
0:89l
D ¼
ð1Þ
b ꢂ cos y

Pd@Ni/MWCNT as a Catalyst
E
(a)
(b)
20 nm
20 nm
Fig. 4. Energy-dispersive X-ray spectroscopy spectra of (a) Ni/multiwalled carbon nanotube (MWCNT) and
(b) Pd@Ni/MWCNT composites The insets in (a) and (b) are the size distribution of the metal particles on Ni/MWCNT
and Pd@Ni/MWCNT derived from TEM images, which were established from the measurements of 100 particles.
2.9
(a)
C
35
Ni/MWCNTs
2.3
Average size ϭ 7.5
Particle number ϭ 100
30
25
20
15
1.8
1.2
0.6
0
Cu
10
5
0
5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5
Particle size [nm]
Ni
Cu
Ni
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
Energy [keV]
5.5
4.4
3.3
2.2
1.1
0
(b)
C
35
Pd@Ni/MWCNTs
Average size ϭ 4.0
Particle number ϭ 100
30
25
20
15
10
5
0
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
Particle size [nm]
Cu
Ni
Cu
Ni
Cu
Pd
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
Energy [keV]
Fig. 5. Transmission electron microscopy images of (a) Pd@Ni nanoparticles on pristine multiwalled carbon
nanotubes (MWCNT) and (b) Pd@Ni nanoparticles on acid-treated MWCNT.

F
M. Zhang et al.
(a)
C (002)
(a) MWCNT
Table 2. Benzyl alcohol oxidation catalyzed by Pd@Ni/multiwalled
carbon nanotubes at different times
Benzyl alcohol (2 mmol), K2CO3 (6 mmol), catalyst (Pd: 0.2 mmol), H2O2,
water, 808C
Ni (111)
(b) Ni/MWCNT
(c) Pd/MWCNT
Ni (200)
Ni (220)
Entry
Time
[h]
Benzyl alcohol
conversion
[%]
Selectivity [%]
Benzaldehyde
Benzoic
acid
Benzyl
Pd (111)
benzoate
Pd (200)
1
2
3
4
5
6
1
2
4
5
6
7
40
59
73
82
99
94
51
64
85
90
98
88
45
33
12
5
4
3
3
5
1
5
Pd (220)
Pd (111)
Ni (111)
Pd (200)
Ni (200)
(d) Pd@Ni/MWCNT
2
Pd (220)
70
7
20
30
40
50
60
80
2θ [deg.]
Table 3. Benzyl alcohol oxidation catalyzed by Pd@Ni/multiwalled
carbon nanotubes at different temperatures
(b)
Pd (111)
Benzyl alcohol (2 mmol), K2CO3 (6 mmol), catalyst (Pd: 0.2 mmol), water,
H2O2, 6 h
Entry Temperature Benzyl alcohol
Selectivity [%]
[8C]
conversion
[%]
Benzaldehyde Benzoic Benzyl
Pd/MWCNT
acid
benzoate
1
2
3
4
5
30
40
60
80
90
75
79
81
99
94
54
62
75
98
90
44
35
21
1
2
3
4
2
3
Pd@Ni/MWCNT
7
selectivity of the aldehyde. However, a further increase of cat-
alyst amount led to a decrease of both alcohol conversion and
aldehyde selectivity. With 0.2 mmol of Pd catalyst, high alcohol
conversion (99 %) and aldehyde selectivity (98 %) can be
achieved.
The reaction times are also investigated, which is shown in
Table 2. The conversion of benzyl alcohol increased with time
and then became steady; however, the selectivity of the alde-
hyde showed a maximum at a time of 6 h. This implies that
increasing the reaction time will not contribute to alcohol
conversion and is not beneficial for selectivity of an aldehyde
product.
The influence of reaction temperature on the benzyl alcohol
conversion and benzaldehyde selectivity is also studied. As
listed in Table 3, the alcohol conversion and aldehyde selectivity
increase with temperature, especially the alcoholconversion,
which could reach as high as 99 % at 808C. However, it should
be noted that if the temperature continued to increase, the
selectivity for benzaldehyde will decrease. So we choose 808C
as the reaction temperature.
40.1
40.2
40.3
2θ [deg.]
40.4
40.5
Fig. 6. Typical X-ray diffraction pattern of (a) multiwalled carbon nano-
tubes (MWCNT), (b) Ni/MWCNT, (c) Pd/MWCNT, and (d) Pd@Ni/
MWCNT.
Table 1. Benzyl alcohol oxidation catalyzed by Pd@Ni/multiwalled
carbon nanotube nanoparticles with different amounts of catalyst
Benzyl alcohol (3 mmol), K2CO3 (6 mmol), water, H2O2, 6 h
Entry
Catalyst
[mmol]
Benzyl alcohol
conversion
[%]
Selectivity [%]
Benzaldehyde Benzoic Benzyl
acid
benzoate
1
2
3
4
5
6
0.05
0.10
0.15
0.18
0.2
31
54
72
89
99
90
56
70
91
91
98
88
38
26
7
6
4
6
2
2
3
7
4
0.22
9
The catalytic performance of the different catalysts for the
oxidation of benzyl alcohol are presented in Table 4. The
catalytic activity of Ni/MWCNT was very poor. Although a
high selectivity with the Pd-rich catalyst (Pd/MWCNT) was
observed, the conversion was obviously lower than for Pd@Ni/
MWCNT catalysts at the same Pd loadings. The enhanced
catalytic activity has been attributed to several factors, including
the interaction between the metal and the MWCNT,[35] d-band
centre shift,[24] and crystal nickel entering into the palladium
crystal lattice leading to a synergistic effect. Zhao et al. used
density functional theory (DFT) calculations to show that extra
initially selected as catalysts for the oxidation of benzyl alcohol.
First, we examine the effect of catalyst amount on the alcohol
conversion and aldehyde selectivity. The products are mainly
aldehyde and acid, while an ester is also found in some cases. As
shown in Table 1, a low amount of catalyst results in low alcohol
conversion, and the products are mostly a mixture of aldehyde
and acid. By increasing the catalyst amount from Pd 0.05 to
0.2 mmol, the alcohol conversion increased and so did the

Pd@Ni/MWCNT as a Catalyst
G
Table 4. Benzyl alcohol oxidation catalyzed by different metal-
deposited multiwalled carbon nanotube (MWCNT) catalysts at the
same conditions
Table 5. Recycling of the Pd@Ni/multiwalled carbon nanotube
catalyst for the oxidation of benzyl alcohol
Benzyl alcohol (3 mmol), K2CO3 (6 mmol), catalyst (Pd: 0.2 mmol), water,
H2O2, 808C, 6 h
Benzyl alcohol (2 mmol), K2CO3 (6 mmol), catalyst (0.2 mmol), water,
H2O2, 808C, 6 h
Cycle number
Conversion [%]
Selectivity [%]
Catalyst
Benzyl alcohol
conversion
[%]
Selectivity [%]
1
2
3
4
99
92
87
84
98
95
96
92
Benzaldehyde Benzoic Benzyl
acid
benzoate
Ni/MWCNT
10
91
99
6
94
98
2
3
1
2
3
1
Pd/MWCNT
Pd@Ni/MWCNT 3
Pd (111)
Pd@Ni/MWCNTs
(a)
(b)
Ni (111)
Pd (200)
Separation
Agitation
C (002)
Ni (200)
Pd (220)
Ni (220)
20
30
40
50
60
70
80
2θ [deg.]
Fig. 7. The states of Pd@Ni/multiwalled carbon nanotube particles:
(a) without a magnetic field and (b) in a magnetic field.
Fig. 8. Typical X-ray diffraction pattern of Pd@Ni/multiwalled carbon
nanotubes (MWCNT) after four catalytic cycles.
Ni–C bonds form at the interface, as such, more electrons
transfer from the interfacial C–C bonds to the Ni–C bonds.[35]
The interaction between Ni atoms and Pd atoms, such as the
space change of the Pd crystal lattice induced by the filling of Ni
atoms, impart Pd@Ni/MWCNT with a fairly conductive
network to allow facile charge-transfer and mass-transfer
processes. Studies have found that the space change of a noble
metal crystal lattice could improve catalytic activity.[24]
characteristic peaks of the catalyst do not show obvious changes
both before and after reaction. Other characteristic peaks such as
for NiO were not visualised by the XRD pattern, as shown in
Fig. 8. With subsequent reuse the selectivity remains almost the
same. Nevertheless, the conversion decreased the catalytic
activity gradually over time. This phenomenon might be attrib-
uted to the partial aggregation of Pd@Ni nanoparticles after
several recycles. In summary, Pd@Ni/MWCNT have relatively
good stability and give good recycle performance.
After oxidation, the reaction solution was transferred into a
vessel and a magnet was placed on the outside of the vessel wall.
It is observed that the magnetic Pd@Ni/MWCNT particles can
be separated from aqueous solution under an external magnetic
field, and can be redispersed into the water with agitation, as
shown in Fig. 7. In the absence of an external magnetic field, the
dispersion of the Pd@Ni/MWCNT particles was visibly dark
and homogeneous (Fig. 7a). When the external magnetic field
was applied, the Pd@Ni/MWCNT particles aggregated, leading
to transparency of the dispersion (Fig. 7b). This phenomenon
reveals that the Pd and MWCNT do not affect the magnetic
properties of the Ni nanoparticles during the synthesis process.
The Pd@Ni/MWCNT catalyst quickly adsorbed onto the wall of
the vessel and the catalyst was then collected and washed with
ethanol and deionised water three times and dried at 608C, for
reuse. The recycling performance of the catalyst is shown in
Table 5. The catalyst exhibited a high catalytic activity with high
conversion and selectivity. In addition, the change in structure
and composition of the Ni after several catalytic cycles of
benzyl alcohol oxidation, determined by XRD patterns, were
compared. From the XRD pattern we can clearly see that the
Conclusions
A magnetically recoverable Pd@Ni bimetallic nanoparticle
catalyst supported on MWCNT was successfully prepared by a
simple method. The as made catalyst gave high catalytic activity
towards the selective oxidation of benzyl alcohol under rela-
tively mild conditions in the presence of H2O2. A green highly
effective route was thus achieved for the transformation of
benzyl alcohol to benzaldehyde and the selective synthesis of
other aromatic or aliphatic aldehydes is also expected. The
results indicated that benzaldehyde could be obtained in a high
yield and selectivity from benzyl alcohol using the Pd@Ni/
MWCNT catalyst in water as a solvent in the presence of H2O2
as an oxygen source at 808C in 6 h. The catalyst can be easily
separated magnetically and a high catalytic activity was retained
after recycling the catalyst four times in comparison with the
pure MWCNT carriers and the Pd noble metal catalysts. It is
implied that Pd nanoparticles supported on Ni/MWCNT are

H
M. Zhang et al.
very promising for portable applications in the selective oxi-
dation of benzyl alcohol to aldehyde and is valuable for the
development of other MWCNT-based metallic catalyst systems.
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Acknowledgements
This work was supported financially by the Jiangsu Science and Technology
Support Program (BE2010144), Zhenjiang industry supporting plan
(GY2010020), and the Research Foundation for Talented Scholars of
Jiangsu University (11JDG149). Dr M. M. Zhang acknowledges the support
of the Innovation Funds of Jiangsu University (CXLX12_0647).
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