Catalytic hydrogenation of acetophenone over shape controlled Pd catalysts supported on sheet-like NiO

Article abstract of DOI:10.1016/j.cattod.2013.06.025

Cubic, truncated octahedral and hexagonal/triangular Pd/NiO supported catalysts were synthesized and applied in the hydrogenation of acetophenone. The catalysts were characterized by XRD, TEM, XPS and TG-DCS, and the catalytic performances for selective h

Full text of DOI:10.1016/j.cattod.2013.06.025

Catalysis Today 216 (2013) 200–204
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Catalytic hydrogenation of acetophenone over shape controlled Pd
catalysts supported on sheet-like NiO
Ye Gou, Xin Liang∗, Biaohua Chen
Department of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Cubic, truncated octahedral and hexagonal/triangular Pd/NiO supported catalysts were synthesized and
applied in the hydrogenation of acetophenone. The catalysts were characterized by XRD, TEM, XPS and
TG–DCS, and the catalytic performances for selective hydrogenation of acetophenone were evaluated
under the condition of 80 ◦C, 3 h and 1.5 MPa pressure of hydrogen in alcohol media without additives.
Results showed that the conversion of 19% and selectivity of 92% were achieved by hexagonal/triangular
Pd/NiO catalyst. And when truncated octahedral Pd/NiO was used as the catalyst, the conversion of
acetophenone was raised to 40%, while the selectivity of 1-phenethanol decreased to 89%. As to cubic
Pd/NiO catalyst, the conversion to further improved to 70% and the selectivity of 1-phenethanol went
down to 80%.
Received 17 March 2013
Received in revised form 24 June 2013
Accepted 26 June 2013
Available online 27 July 2013
Keywords:
Hydrogenation
Shape and size
Pd nanocrystals
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
tionship for the Pd based catalysts is desirable for designing and
developing catalysts with enhanced catalytic performance.
The hydrogenation of acetophenone has a great significance
in the synthesis of pharmaceuticals and fragrances. The pos-
sibly products of the hydrogenation of acetophenone include
1-phenylethanol, ethyl benzene and cyclohexyl ethanol. Among
these products, phenylethanol is of great importance since it is
an important chemical intermediates [1]. Pd-based catalysts have
been widely used for hydrogenation process for their strong inter-
action with H2 molecules, their high activity, and the mild reaction
conditions required [2–9]. It is well documented that the activ-
ity and selectivity of Pd based catalysts can be influenced by the
size and shape of Pd nanoparticles. Recently, it is reported that
the activity of the Pd catalysts can be effectively tailored by the
use of nanocrystals enclosed by specific crystal facets that are
intrinsically more active for a particular reaction [10–13]. Since
the exposed facets of designed nanocrystals are determined by its
shape, an exquisite shape control of Pd nanocrystals is therefore
highly desired for tailoring their catalytic properties and also a pre-
requisite for high performance in various catalytic applications. For
example, Pd cubes can display only (1 0 0) facets on the surface,
while hexagonal plates mainly present (1 1 1) facets and truncated
octahedrons have a mixture of both. For structure–sensitive reac-
tions, such shape and size effects can greatly influence the catalysts’
performance. Therefore, investigating the structure–activity rela-
Supported palladium catalysts, whose catalytic performance
can be modified by the nature of support and the metal-support
interaction, are extensively applied for the efficient utilization of
precious metals and relative low cost. A large number of researches
have reported that the catalytic behavior of Pd in a reductive atmo-
sphere is significantly modified when metal oxides such as MnOx,
NiO, VOx, TiO2, Fe3O4 and MoOx are used as a support [14]. Among
these oxides, NiO is an important inorganic material which has
been widely applied in the field of industrial catalysis. Porous NiO
nanosheets can serve as an ideal catalyst support which can effec-
tively facilitate the dispersion of the Pd particles for the large
specific surface area, unique porous structure and good crystallinity
[15,16].
In this study, uniform Pd nanocrystals with cubic, truncated
octahedral, and hexagonal/triangular shapes were synthesized
through a solution-phase method with poly(vinyl pyrrolidone)
(PVP) serving as a stabilizer. Then, these Pd nanocrystals with well-
defined shapes were supported on porous NiO nanosheets and
tested as catalysts in the alcohol-assisted selective hydrogenation
of acetophenone. A comparative study was carried out in order
to investigate their structural-activity relationship of these Pd/NiO
catalysts.
2. Materials and methods
2.1. Materials
∗
All the chemicals were of analytical grade and used as
received without further purification. Ni(NO3)3·6H2O, oleylamine,
Corresponding author. Tel.: +86 10 64412054.
E-mail address: liangxin@mail.buct.edu.cn (X. Liang).
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2013.06.025

Y. Gou et al. / Catalysis Today 216 (2013) 200–204
201
poly(vinylpyrrolidone) (PVP), KBr, ascorbic acid (AA) and abso-
3. Characterization
lute ethanol were supplied by Beijing Chemical Reagent Company.
Na2PdCl4 was supplied by Alfa Aesar.
Powder X-ray powder diffraction (XRD) was performed on
a Bruker D8-Advance X-ray diffractometer with CuK␣ radia-
tion (ꢀ = 0.15406 nm). The morphology and size of the samples
were characterized on a transmission electron microscope (H800
Hitachi). UV–vis spectroscopy spectra were obtained by SHIMADZU
UV-3600 Ultraviolet visible near infrared spectrophotometer. Ther-
mogravimetric (TG) analysis and differential scanning calorimetric
(DSC) measurements were performed using a Setaram Setsys EV
24 thermal analyzer and O2–He mixture (20 mL min−1, 25% O2 by
volume) as a carrier gas at a heating rate of 10 ◦C min−1 in a range
of 100–600 ◦C. The chemical composition was confirmed by X-ray
photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250
Xi). The catalytic experiments were carried out on a 50 mL stainless
steel autoclave. The stirring effect was preliminarily investigated
and a stirring rate of 400 rpm was employed, which turned out to
be sufficient to eliminate the diffusion limit. In the hydrogenation of
acetophenone, 0.05 g catalyst (dried at 80 ◦C) was transferred into
the solution composed of 2.5 mL ethanol and 0.025 ml acetophe-
none. After replacing the air with H2 in the reactor, the reaction was
performed at 80 ◦C and 1.5 Mpa of H2 pressure for 3 h. The hydro-
genation product was filtered to remove the solid catalyst and was
analyzed by a gas chromatograph equipped with a flame ionization
detector.
2.2. Synthesis
For the synthesis of porous NiO nanosheets, 0.291 g mixture of
Ni(NO3)2·6H2O was dissolved in 10 mL ethanol and 10 mL H2O to
form solution A. Two millilitres oleylamine was added to 10 mL
ethanol to form solution B. Solution A was mixed with solution
B under strong agitation. Then, the mixture was transfered to a
50 mL autoclave. The autoclave was sealed and heated at 180 ◦C
for 12 h. As the autoclave cooled to room temperature naturally,
the supernatant solution was poured out, and the samples could be
directly collected from the bottom of the autoclave. The samples
were washed, dried and calcined in a muffle furnace at 400 ◦C for
2 h to obtain NiO [15].
For the synthesis of Pd nanocubes [17], 8.0 mL of an aqueous
solution containing 0.105 g PVP, 0.060 g AA and 0.30 g KBr was
placed in a 100 mL three-flask, and pre-heated in air under mag-
netic stirring at 80 ◦C for 10 min. Then, 3.0 mL of an aqueous solution
containing 0.057 g Na2PdCl4 was added. After the flask had been
capped, the reaction was allowed to proceed at 80 ◦C for 3 h, The
product was collected by centrifugation and washed 10 times with
water to remove excess PVP. For the synthesis of Pd truncated octa-
hedrons, the synthetic procedure was similar as the synthesis of
Pd cubes, except that no KBr was added. For the synthesis of Pd
hexagonal/triangular plates, PVP served as both reducing agent and
stabilizer without AA and KBr in the reaction.
4. Results and discussion
Palladium nanocubes, truncated octahedrons, and hexag-
1% Pd/NiO supported catalysts were prepared by impregna-
tion method, 0.1 g NiO nanosheets were added into dispersed
alcohol solution involved 0.001 g Pd nanocrystals with vigorous
magnetic stirring. Then, Pd/NiO catalysts were collected by fil-
tration, washed with ethanol, and dried in an oven overnight at
80 ◦C.
onal/triangular nanoplates have been synthesized through
a
reported PVP-assisted approach [16–18]. PVP can be used as an
ideal reducing agent and surfactant in the synthetic process. Thin
plates with hexagonal/triangular shapes are obtained at a low
reduction rate of Pd2+, in which both nucleation and growth
behaviors of Pd nanocrystals mainly follow a thermodynamically
Fig. 1. TEM images of Pd nanocrystals and Pd/NiO catalysts, (a) hexagonal/triangular plates, (b) cubes, (c) truncated octahedrons, (d) hexagonal/triangular Pd/NiO, (e)cubic
Pd/NiO, (f) truncated octahedrad Pd/NiO.

202
Y. Gou et al. / Catalysis Today 216 (2013) 200–204
Fig. 2. The size distribution of different Pd nanocrystals, (a) hexagonal/triangular plates, (b) cubes, (c) truncated octahedrons.
controlled pathway. For the synthesis of truncated octahedrons and
cubes, l-ascorbic acid is used as a reducing agent to ensure the fast
reduction of Na2PdCl4 precursor. KBr is used as a capping agent
in promoting the (1 0 0) facets to selectively obtain Pd nanocubes,
while truncated octahedrons formed at the absent of KBr.
Fig. 1a–c shows the TEM images of as-prepared Pd nanocrystals
with various morphologies, such as cubic, truncated octahedral and
hexagonal/triangular morphologies. Fig. 1a shows typical Pd hexag-
onal/triangular nanoplates with uniform morphology and narrow
size distribution. It can be calculated that the particle size distri-
bution of Pd hexagonal/triangular nanoplates is 50–100 nm with
an average of ca. 82 nm. As the crystal structure model shown in
the insert of Fig. 1a, these hexagonal/triangular nanoplates can be
viewed as plates with both top and bottom faces covered by the
(1 1 1) facets.
The morphology of Pd nanocubes is shown in Fig. 1b. These Pd
nanocrystals show uniform cubic morphology with an average side
length of 9.3 nm (Fig. 2). The Pd nanocubes are enclosed by six (1 0 0)
facets. The morphology of Pd truncated octahedrons is shown in
Fig. 1c and the average particle size is 9.1 nm. The crystal model
reveal that the truncated octahedrons expose both (1 1 1) and (1 0 0)
facets.
The three kinds of Pd nanocrystals are supported on porous
sheet-like NiO. The NiO nanosheets have a good absorption for
Pd nanoparticles for their porous structure, high surface area
and the electrostatic interaction. When Pd nanocrystals and NiO
nanosheets are mixed together, Pd nanocrystals can be easily
absorbed on the surface of NiO. Fig. 1d–f shows the TEM images
of as-prepared Pd/NiO supported catalysts. TEM images confirm
that the Pd nanoparticles with various morphologies are well dis-
persed on the surface of porous NiO nanosheets. It is worthy to
noting that the dispersion of Pd hexagonal/triangular nanoplates
on NiO is worse than that of Pd nanocubes and truncated octahe-
drons, which can be caused by the relatively bigger particles size of
Pd hexagonal/triangular nanoplates.
two peaks located at the region of 300–400 ◦C exhibit the loss
of physical and chemical absorbed small organic molecules. The
third peak located at the region of 400–600 ◦C is corresponding
to the combustion of PVP [19]. Careful observation reveals that
there is a big variation for the third PVP combustion peak over
different Pd/NiO catalysts. It can be found that the combustion
temperature of PVP increases in the order of hexagonal/triangular
Pd/NiO > truncated octahedral Pd/NiO > cubic Pd/NiO. The begin-
ning temperatures of PVP oxidation are 411 ◦C, 421 ◦C and 440 ◦C for
cubic, truncated octahedral and hexagonal/triangular Pd/NiO cat-
alysts, respectively. This loss stage can be viewed as the oxidation
of PVP, and Pd nanocrystals act as catalysts in the reaction of PVP
oxidation. This variation indicates that the shape of Pd nanocrystals
can greatly influence the combustion temperature of the covered
PVP. Pd nanocubes show better catalytic capacity than truncated
octahedrons and hexagonal/triangular plates.
The XPS spectra of Pd/NiO supported catalysts in the Pd 3d
region are shown in Fig. 5. Two typical peaks with binding energy
at the regions of 332.0–338.0 eV and 340.0–345.0 eV are observed
in the spectra, which can be well assigned to the electron tran-
sitions of Pd 3d5/2 and Pd 3d3/2 [20]. The peaks of Pd 3d5/2 of
these Pd/NiO catalysts located at around 335 eV, which can be
attributed to Pd0 species on the surface. Careful observation on
the peaks of Pd 3d5/2 reveals there is a slight variation on the Pd
3d5/2 peaks for Pd nanocrytals with different morphologies. The Pd
3d5/2 binding energies are 334.6 eV, 335.1 eV and 335.1 eV for Pd
hexagonal/triangular nanoplates, nanocubes and truncated octa-
hedrons, respectively. The higher Pd 3d5/2 binding energy for Pd
truncated octahedrons and cubes may be caused by the size effect.
Because of the small size, the charges transfer from Pd to the cap-
ping agent PVP easily for Pd truncated octahedron and cube, which
hexagonal/triangular Pd/NiO
truncated octahedral Pd/NiO
cubic Pd/NiO
Fig. 3 shows XRD patterns of the Pd/NiO supported catalysts. The
XRD patterns can be indexed to the face-centered cubic NiO phase
(JCPDS No. 71-1179). And the peaks locate at 37.13◦, 43.11◦, and
62.62◦ are well corresponding to (1 1 1), (1 0 0), and (2 2 0) facets.
For the samples of Pd nanocubes and truncated octahedrons, no
diffractive peaks from Pd nanocrystals are observed in the XRD
patterns, revealing the good dispersity of active Pd nanocrystals
on the surface of NiO. For Pd hexagonal/triangular nanoplates sup-
ported NiO catalyst, weak diffractive peaks from Pd nanocrystals
are observed in the XRD pattern.
TG–DSC curves of different Pd/NiO catalysts have been shown
in Fig. 4. The whole weight loss process can be parted into two
stages. The first weight loss stage is 100–400 ◦C, corresponding to
the loss of absorbed small organic molecules. The second stage is
in the region of 400–600 ◦C, corresponding to the combustion of
PVP. There are three peaks observed in the DSC curves. The first
20
30
40
50
60
70
80
2Theta(degree)
Fig. 3. The XRD patterns of as-synthesized Pd/NiO catalysts.

Y. Gou et al. / Catalysis Today 216 (2013) 200–204
203
100
100
95
90
85
80
75
80
60
40
20
0
100
a
b
98
DSC Exothermic
weight
DSC Exothermic
weight
80
60
40
20
0
96
94
92
90
88
86
84
82
-20
100
100
200
300
400
500
600
200
300
400
500
600
Temperature (ºC)
Temperature (ºC)
80
c
102
100
98
70
60
50
40
30
20
10
DSC Exothermic
weight
96
94
92
90
88
86
84
0
100
200
300
400
500
600
Temperature (ºC)
Fig. 4. TG–DSC curves of Pd/NiO catalysts, (a) cubic Pd/NiO catalyst, (b) truncated octahedral Pd/NiO catalyst, (c) hexagonal/triangular Pd/NiO catalyst.
leads to a high binding energy. The different electronic structures of
Pd nanoparticles with various sizes may exert the electronic effect
on their catalytic performance [21]. Table 1 lists the surface Pd
atomic percentage of different Pd/NiO samples calculated based on
the XPS spectra. Pd atomic percentages are 1.00%, 0.85% and 0.92%
for the samples of Pd hexagonal/triangular nanoplates, truncated
octahedrons and nanocubes, respectively. According to experimen-
tal section, the load quantity of Pd in the catalysts is 1 mol%. The XPS
results confirm that most of Pd nanoparticles are absorbed by NiO
porous nanosheets.
The catalytic performances of these Pd/NiO supported cata-
lysts are evaluated by the hydrogenation of acetophenone reaction.
Fig. 6 shows the comparative results of the catalytic activity
and selectivity of the three kinds of Pd/NiO catalysts under the
designed condition of 80 ◦C, 3 h and 1.5 MPa pressure of H2.
It is clear that all of the samples have good selectivities to
1-phenylethanol: 80% for cubic Pd/NiO, 89% for truncated octa-
hedral Pd/NiO and 92% for hexagonal/triangular Pd/NiO, which
best demonstrates that Pd nanoparticles are good catalysts for
selective hydrogenation of acetophenone. It is also observed that
the catalytic activity of these three catalysts follows an order of
hexagonal/triangular Pd/NiO > truncated octahedral Pd/NiO > cubic
Pd/NiO. It can be found that the conversion of 19% is achieved
by hexagonal/triangular Pd/NiO, which is smaller than that of
truncated octahedral catalyst (40%) and cubic catalysts (70%).
According to the selectivity and conversion results, it is clear that
Pd nanocubes show best yield to 1-phenylethanol. This result can
be attributed to the size effect. The average size of Pd hexago-
nal/triangular plates is much bigger than that of Pd cubes and
truncated octahedrons. Obviously, the smaller Pd nanocrystals can
334.6
hexagonal/triangular Pd/NiO
Pd(0)
truncated octahedral Pd/NiO
cubic Pd/NiO
335.1
335.1
Table 1
The average size (dTEM), Pd 3d5/2 binding energy (B.E. of Pd 3d5/2) and surface Pd
atomic percentage (Pd surface%) estimated from XPS.
Sample
dTEM (nm)
B.E. of Pd 3d5/2 (eV)
Pd surface (%)
325
330
335
340
345
350
355
360
365
Hexagonal/triangular
Truncated octahedron
Cube
82
9.1
9.3
334.6
335.1
335.1
1.00
0.85
0.92
Binding Energy (eV)
Fig. 5. Pd 3d XPS spectra of Pd/NiO catalysts.

204
Y. Gou et al. / Catalysis Today 216 (2013) 200–204
Pd/NiO catalysts are fully characterized by various analysis meth-
conversion of acetophenone
selectivity to phenethanol
selectivity to phenylethane
ods, such as XRD, TEM, XPS, and TG–DCS. The comparative study
of the catalytic performances for hydrogenation of acetophe-
none over these catalysts shows that Pd nanocubes show better
yield to 1-phenylethanol than truncated octahedrons and hexago-
nal/triangular plates, indicating that the catalytic activity of Pd/NiO
catalysts is strongly depended on the size and shape of Pd nanocrys-
tals. This research not only gives a suggestion for hydrogenation of
acetophenone to phenethanol under mild conditions, but also pro-
vides important experiment data for catalyst design on size and
shape.
90
80
70
60
50
40
30
20
10
0
Acknowledgments
This work was financially supported by NSFC (Grant No.
21001015, 20821004), RFDP (Grant No. 20100010120003) and the
State Key Project of Fundamental Research for Nanoscience and
Nanotechnology (2011CB932402).
hexagonal/triangular
truncated octahedron
cube
Fig. 6. The comparison of the catalytic performances for the hydrogenation of ace-
tophenone over there Pd/NiO catalysts.
References
provide more catalytic active sites for the hydrogenation reaction.
For the truncated octahedral and cubic Pd/NiO catalysts within sim-
ilar particle size distribution, cubic Pd/NiO catalyst shows better
catalytic activity than the truncated octahedral catalyst. It is well
known that Pd nanocubes can only expose (1 0 0) facets but trun-
cated octahedrons can expose both (1 1 1) and (1 0 0) facets. As Pd
nanocrystals adopt a face centered cubic (fcc) crystal structure, the
surface energy of (1 0 0) crystal planes is bigger than (1 1 1) crys-
tal planes. The catalytic difference of these two catalysts can be
attributed to the shape effect. Xia and his co-workers have docu-
mented that there are two types of active sites involved in the Pd
nanocubes and truncated octahedrons: atoms on the planes and
atoms at the edges, and overhydrogenation occurred mainly at the
edge sites [22]. At the same size level, Pd cubes have more active
edge sites than truncated octahedrons. Thus cubes have better cat-
alytic activity than truncated octahedrons and can lead to a further
hydrogenation of 1-phenethanol to ethylbenzene easily. As a result,
Pd nanocubes show best yield to 1-phenylethanol, but the selectiv-
ity of 1-phenylethanol over cubic Pd/NiO is worse than truncated
octahedral Pd/NiO.
[1] C.S. Chen, H.W. Chen, Appl. Catal. A Gen. 2 (2004) 207–213.
[2] G.F. Santori, A.G. Moglioni, V. Vetere, G.Y. Moltrasio Iglesias, M.L. Casella, O.A.
Ferretti, Appl. Catal. A Gen. 269 (2004), 215-215.
[3] C. Chen, H. Chen, W. Cheng, Appl. Catal. A Gen. 248 (2003) 117–128.
[4] L. Cerveny, Z. Belohlav, M.N.H. Hamed, Res. Chem. Intermed. 22 (1996) 15–22.
[5] M. Casagrande, L. Storaro, A. Talon, M. Lenarda, R. Frattini, P. Maire-les-Torres,
J. Mol. Catal. A Chem. 188 (2002) 133–139.
[6] M.A. Aramendia, V. Borau, J.F. Gomez, A. Herrera, C. Jimenez, J.M. Marinas, J.
Catal. 140 (1993) 335–343.
[7] A. Drelinkiewicza, A. Waksmundzka, W. Makowski, J.W. Sobczak, A. Krol, A.
Zieba, Catal. Lett. 94 (2004) 143–157.
[8] M. Bejblova, P. Zamostny, L. Cerveny, J. Cejka, Collect. Czech. Chem. Commun.
68 (2003) 169–175.
[9] J.M. Bonnier, J.P. Damon, J. Masson, Appl. Catal. 42 (1988) 285–297.
[10] R. Narayanan, M.A. El-Sayed, Nano Lett. 4 (2004) 1343–1348.
[11] S.E. Habas, H. Lee, V. Radmilovic, G.A. Somorjai, P. Yang, Nat. Mater. 6 (2007)
692–697.
[12] K.M. Bratlie, H. Lee, K. Komvopoulos, P. Yang, G.A. Somorjai, Nano Lett. 7 (2007)
3097–3101.
[13] C. Wang, H. Daimon, T. Onodera, T. Koda, S. Sun, Angew. Chem. Int. Ed. 47 (2008)
3588–3591.
[14] A. Zieba, A. Krol, J.W. Sobczak, Catal. Lett. 94 (2004) 143–156.
[15] X. Liang, J.J. Xiao, Y. Gou, B.H. Chen, J. Mater. Res. 26 (2011) 3091–3097.
[16] J.J. Xiao, B.H. Chen, X. Liang, Catal. Sci. Technol. 1 (2011) 999–1005.
[17] Y.N. Xia, J.Y. Liu, M.S. Jin, H.Y. Liu, Nano Res. 4 (2011) 83–91.
[18] B. Lim, M.J. Jiang, J. Tao, Adv. Funct. Mater. 19 (2009) 189–200.
[19] Z.F. Wang, B. Shen, N.Y. He, Mater. Lett. 58 (2004) 3652–3655.
[20] M. Burn, A. Berthet, J.C. Bertolini, J. Electron. Spectrosc. 104 (1999)
55–60.
5. Conclusion
[21] H. Tsunoyama, N. Ichikuni, H. Sakurai, T. Tsukuda, J. Am. Chem. Soc. 131 (2009)
7086–7093.
[22] M.C. Quesada, A. Yarulin, M.S. Jin, Y.N. Xia, L.K. Minsker, J. Am. Chem. Soc. 133
(2011) 12787–12794.
Cubic, truncated octahedral and hexagonal/triangular Pd/NiO
catalysts are synthesized and applied in the hydrogenation of
acetophenone. The physicochemical properties of as-prepared