Preparation of nano-Ni/meso-Ce-TiO2 by one-step in a sol-gel system and its catalytic performance for hydrogenolysis of xylitol

Article abstract of DOI:10.1039/c5ra08472e

The conventional method for the preparation of metal nanoparticles loaded on a mesoporous metal oxide is tedious, and its chemical and physical properties cannot be optionally controlled. A simple and convenient method by one-step in the AcHE sol-gel system was employed for the preparation of nano-Ni/meso-Ce-TiO₂ samples with different Ni nanoparticle loadings. Their physical and chemical properties were characterized by XRD, liquid N₂ adsorption-desorption, EDX, ICP, FT-IR, H₂-TPR and TEM techniques, and their catalytic performance was investigated in the hydrogenolysis of xylitol. The results show that these samples with ordered mesoporous structures, uniform pore size and large specific surface area can be obtained. The Ni nanoparticles can be well dispersed on the mesoporous Ce-TiO₂ surface. The stability of the Ni nanoparticles on the mesoporous TiO₂ would be promoted by the introduction of a small amount of cerium species. The 8% nano-Ni/meso-Ce-TiO₂ sample shows a higher catalytic performance than other samples. The conversion of xylitol was 87.9% and a total yield of diols of 83.2% can be reached. It shows high stability, and the conversion of xylitol and the total yield of diols decreased slightly after repeating the reaction 8 times. The method provides a valuable reference for the preparation of metal nanoparticles loaded on a mesoporous metal oxide, especially for non-noble metal nanoparticles.

Full text of DOI:10.1039/c5ra08472e

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Preparation of nano-Ni/meso-Ce–TiO2 by one-
step in a sol–gel system and its catalytic
performance for hydrogenolysis of xylitol
Cite this: RSC Adv., 2015, 5, 70410
a
a
b
a
a
*
Zhiwei Zhou, Songshan Dai, Juan Qin, Pengcheng Yu and Wenliang Wu
The conventional method for the preparation of metal nanoparticles loaded on a mesoporous metal oxide
is tedious, and its chemical and physical properties cannot be optionally controlled. A simple and
convenient method by one-step in the AcHE sol–gel system was employed for the preparation of nano-
Ni/meso-Ce–TiO2 samples with different Ni nanoparticle loadings. Their physical and chemical
properties were characterized by XRD, liquid N2 adsorption–desorption, EDX, ICP, FT-IR, H2-TPR and
TEM techniques, and their catalytic performance was investigated in the hydrogenolysis of xylitol. The
results show that these samples with ordered mesoporous structures, uniform pore size and large
specific surface area can be obtained. The Ni nanoparticles can be well dispersed on the mesoporous
Ce–TiO2 surface. The stability of the Ni nanoparticles on the mesoporous TiO2 would be promoted by
the introduction of a small amount of cerium species. The 8% nano-Ni/meso-Ce–TiO2 sample shows a
higher catalytic performance than other samples. The conversion of xylitol was 87.9% and a total yield of
diols of 83.2% can be reached. It shows high stability, and the conversion of xylitol and the total yield of
diols decreased slightly after repeating the reaction 8 times. The method provides a valuable reference
for the preparation of metal nanoparticles loaded on a mesoporous metal oxide, especially for non-
noble metal nanoparticles.
Received 7th May 2015
Accepted 11th August 2015
DOI: 10.1039/c5ra08472e
www.rsc.org/advances
successfully prepare many multicomponent mesoporous metal
oxides.13,14
1. Introduction
Metal nanoparticles have attracted extensive attention due to
their smaller particle size and larger specic surface area, which
are oen desired for wide applications in catalysis, electricity,
magnetic and optics regions.15–18 To improve their catalytic
performance, these metal nanoparticles would be dispersed by
impregnation or deposition on the carriers including zeolites,
mesoporous materials and metal oxides etc.19–22 Unfortunately,
it is hard for the prepared catalysts to work well in practice
owing to the aggregation of the pre-synthesized metal nano-
particles. Compared to the catalysts prepared by impregnation
or deposition, the pre-synthesized noble metal nanoparticles
(Au, Pt and Pd etc.) can be dispersed uniformly on the meso-
porous metal oxides (SiO2, ZrO2, Al2O3 and TiO2 etc.) in the
AcHE sol–gel system, and the structure of the mesoporous
metal oxides would not be affected. Therefore, the method can
improve the utilization of noble metal nanoparticles23 and
would provide a new idea for the preparation of nickel based
catalysts.
Metal oxides have been widely applied in catalysis because of
their acid–base, electrical conductivity and oxidation–reduction
qualities.1 Compared to traditional metal oxides, mesoporous
metal oxides are more suitable as a catalyst carrier owing to
their uniform pore system, which can restrain the aggregation
of active constituents dispersed on their surface.2,3 The meso-
porous metal oxides can be successfully templated from nano-
crystalline, mesoporous carbon materials, silane-functionalized
polymers, organic surfactants and cationic polymers etc.4–10 The
solvent evaporation induced self-assembly process raised by
Sanchez et al. was regarded as the most effective method for the
preparation of mesoporous metal oxides because the inorganic
condensation kinetics of non-silicate mesostructures can easily
be controlled.11,12 According to previous reports, a general sol–
gel system, including acetic acid, hydrochloric acid and ethanol,
which is denoted as the AcHE system, has been proven to
Nickel based catalysts, which were conventionally prepared
by impregnation or deposition, are widely used as hydrogena-
tion catalysts because of their low price and high activity.
Compared to noble metal nanoparticles, the Ni nanoparticles
are difficult to obtain because the precursor is more difficult to
restore and they would gather easily. Up to now, the reports
aChemical Engineering and College of Chemistry and Chemical Engineering, Nanjing
Tech University (formerly: Nanjing University of Technology), Nanjing, 210009, P. R.
China. E-mail: wwl@njtech.edu.cn
bTechnology and Finance Service Center of Jiangsu Province, Productivity Center of
Jiangsu Province, Nanjing 210042, People’s Republic of China. E-mail:
qinjuan_1981@163.com
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about Ni nanoparticles, whose average particle size is less than
6 nm, are quite few.24–26 On the other hand, pre-synthesized Ni
nanoparticles would be oxidized and assembled aer calcina-
tion in the AcHE sol–gel system, and the average particle size of
the Ni nanoparticles would not be controlled accurately.
Therefore, it is most crucial to develop a method for the prep-
aration of Ni nanoparticles loaded on mesoporous metal oxides
without pre-synthesized Ni nanoparticles, and it is still rarely
reported so far.
Diols, such as ethylene glycol (EG), 1,2-propylene glycol (1,2-
PG) and 1,3-propylene glycol (1,3-PG), are widely used as phar-
maceuticals, liquid fuels, antifreeze agents and solvents.27
However, the diols are industrially obtained from petroleum
feedstocks by the hydration of ethylene oxide or propylene
oxide. The catalytic transformation of polyols to diols has
attracted considerable attention.
Fig. 2 XRD patterns for the (a) 4% nano-Ni/meso-TiO2, (b) 8% nano-
Ni/meso-TiO2, (c) 12% nano-Ni/meso-TiO2, (d) 16% nano-Ni/meso-
TiO2, and (e) meso-TiO2 samples.
In this work, the Ni nanoparticles uniformly loaded on the
ordered mesoporous Ce–TiO2 samples prepared by one-step in
the AcHE sol–gel system were rstly reported, and the chemical
and physical properties were characterized. Their catalytic
performances in the hydrogenolysis of xylitol were investigated.
Fig. 1 is the schematic diagram for the preparation of nano-
Ni/meso-Ce–TiO2 samples by one-step. This facile method is
based on a sol–gel process combined with the AcHE system
using F127 as the template. In the micellar solution of the
titanium precursor, the Ni and Ce precursors could be well
dispersed, and F127 can cross-link and grow slowly with inor-
ganic precursors in the aging and solvent evaporation
processes. Aer calcination in air and reduction in a hydrogen
atmosphere, the Ni and Ce could be directly incorporated into
the mesoporous TiO2. However, the Ni nanoparticles would be
adsorbed on the surface of the mesoporous TiO2 prepared by
impregnation, which always results in their aggregation or loss
during its application in some reactions.
tetrabutyl titanate as the Ti source. In a typical synthesis
process, 1.6 g F127, 1.2 g concentrated hydrochloric acid and 2.4
g acetic acid were diluted into 30 ml anhydrous ethanol, and the
solution was stirred at 303 K for 10 min to form a transparent
solution. Aer that, 3.4 g tetrabutyl titanate was added into the
above solution and stirred for another 2 h at 303 K. The solution
was transferred to a Petri dish to evaporate the ethanol at 313 K,
and the residual lm was aged for 24 h at 338 K. The sample was
calcined in air at 623 K for 5 h to remove the organic template
and dened as meso-TiO2.
In order to prepare the nano-Ni/meso-Ce–TiO2 samples,
nickel nitrate (Ni(NO3)2$6H2O) and cerium nitrate
2. Experimental
2.1. Sample preparation
The mesoporous TiO2 sample was synthesized according to the
literature23 using F127 as a structure-directing agent and
Fig. 3 (A) Wide-angle XRD patterns, (B) low-angle XRD patterns, (C) N2
adsorption–desorption isotherms and (D) pore size distributions for
the (a) meso-Ce–TiO2, (b) 2% nano-Ni/meso-Ce–TiO2, (c) 4% nano-
Fig. 1 Schematic diagram for the preparation of the nano-Ni/meso- Ni/meso-Ce–TiO2, (d) 6% nano-Ni/meso-Ce–TiO2, and (e) 8% nano-
Ce–TiO2 sample.
Ni/meso-Ce–TiO2 samples.
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Table 1 Physical properties for (a) meso-Ce–TiO2, (b) 2% nano-Ni/
meso-Ce–TiO2, (c) 4% nano-Ni/meso-Ce–TiO2, (d) 6% nano-Ni/
meso-Ce–TiO2, and (e) 8% nano-Ni/meso-Ce–TiO2
Samples
Dp (nm)
SBET (m2 gꢀ1
)
Vp (cm3 gꢀ1
)
Loading (wt%)
a
b
c
d
e
5.1
5.0
4.9
4.9
4.8
214.7
180.4
155.0
147.6
137.5
0.30
0.28
0.26
0.25
0.23
0.0
1.99
4.01
5.97
7.84
Fig. 6 H2-TPR profiles for (a) 8% nano-Ni/meso-TiO2 calcined at 623
K, and 8% nano-Ni/meso-Ce–TiO2 under different calcination
temperatures: (b) 623 K, (c) 673 K, (d) 723 K, (e) 773 K, (f) 823 K and (g)
8% Ni/meso-Ce–TiO2 (IMP).
Fig. 4 XRD patterns for 8% nano-Ni/meso-Ce–TiO2 under different
calcination temperatures: (a) 623 K, (b) 673 K, (c) 723 K, (d) 773 K, (e)
823 K and (f) 8% nano-Ni/meso-TiO2 calcined at 623 K.
Table 2 Physical properties for 8% nano-Ni/meso-Ce–TiO2 under
different calcination temperatures: (a) 623 K, (b) 673 K, (c) 723 K, and (d)
773 K
Samples
Dp (nm)
SBET (m2 gꢀ1
)
Vp (cm3 gꢀ1
)
DNi (nm)
a
b
c
6.2
4.8
7.1
8.1
126.6
137.5
123.0
105.0
0.23
0.23
0.22
0.20
10.2
8.5
16.6
21.2
Fig. 7 The SEM photo (a), and electron mapping of particles (b–d), for
the 8% nano-Ni/meso-Ce–TiO2 sample calcined at 673 K.
d
Fig.
8 TEM images for the 8% nano-Ni/meso-Ce–TiO2 sample
calcined at 673 K.
Fig. 5 Liquid N2 adsorption–desorption isotherms (A) and pore size
distribution (B) for 8% nano-Ni/meso-Ce–TiO2 under different calci-
nation temperatures: (a) 623 K, (b) 673 K, (c) 723 K, and (d) 773 K.
with nickel nitrate and cerium nitrate were added into the
(Ce(NO3)2$6H2O) were used as the Ni and Ce sources, respec- solution at the same time. All the samples were reduced under a
tively. The nano-Ni/meso-Ce–TiO2 samples were prepared under hydrogen atmosphere at an appropriate temperature except for
the same conditions as the meso-TiO2, while tetrabutyl titanate the samples for H2-TPR characterization.
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Fig. 9 FT-IR spectra for (a) meso-TiO2, (b) meso-Ce–TiO2, (c) 8%
nano-Ni/meso-TiO2, and (d) 8% nano-Ni/meso-Ce–TiO2.
Fig. 10 Catalytic stability for the 8% nano-Ni/meso-Ce–TiO2 and 8%
Ni/meso-Ce–TiO2 (IMP) catalysts in the hydrogenolysis of xylitol.
For comparison, a sample, dened as 8% Ni/meso-Ce–TiO2
(IMP), was prepared by the impregnation method.
each measurement. The sample was pretreated in the argon
stream at 623 K for 3 h and then cooled down to room
temperature. Aer that, the mixture of argon and hydrogen was
introduced. The temperature was raised from 313 K to 1073 K at
a rate of 5 K minꢀ1. The consumption of H2 in the reactant
stream was recorded by a thermal conductivity detector (TCD).
2.2.6. TEM measurement. The morphology of the samples
was visualized using a JEOL JEM 2100 transmission electron
microscope (TEM) operated at 120 kV. The samples were
dispersed in ethanol assisted by an ultrasonic technique.
2.2.7. EDX measurement. The dispersion of the semi-
quantitative elemental composition (Ti, Ce, Ni) was veried by
energy dispersive X-ray (EDX) spectrometer analysis in the
Oxford INCA EDAX Detecting Unit.
2.2. Material characterization
2.2.1. X-ray powder diffraction analysis. X-ray powder
diffraction patterns of the samples were obtained on a Bruker
D8 instrument with Ni-ltered Cu Ka radiation (l ¼ 0.154 nm)
and operated at 40 kV and 30 mA. The scanning rate was 0.05ꢁ
sꢀ1 in the 2q range from 0.7ꢁ to 5ꢁ (low-angle XRD) and from 10ꢁ
to 70ꢁ (wide-angle XRD).
2.2.2. Nitrogen adsorption–desorption analysis. Nitrogen
adsorption–desorption isotherms of the samples were
measured at 77 K using a Micromeritics Tristar 3000 system.
The surface area was calculated using the Brunauer–Emmett–
Teller (BET) method in the partial pressure (P/P0) range from
0.01 to 0.99. The mesoporous size distribution was obtained
2.3. Catalytic reaction
from the adsorption branch of the isotherm using the Barrett– The catalytic hydrogenolysis of xylitol was carried out in a 100
Joyner–Halenda (BJH) adsorption model. ml stainless steel autoclave equipped with a magnetically driven
2.2.3. ICP-OES analysis. The loading of Ni nanoparticles impeller. 0.5 g catalyst was added into the solution of 7.5 g
was analyzed by inductively coupled plasma (ICP, Opti- xylitol and 50 ml deionized water. The reactor was ushed
ma2100DV, PerkinElmer, USA) aer the sample had been dis- multiple times with hydrogen to replace air. The reaction was
solved in HNO3 solution.
carried out at 513 K and 5 MPa for 4 h at a stirring speed of 1000
2.2.4. Fourier transform infrared analysis. Fourier trans- rpm. At completion of the reaction, the reactor was cooled down
form infrared spectra were obtained on a Thermo Nicolet to room temperature and the products were collected. The
NEXUS spectrometer with KBr pellets at room temperature.
xylitol was analyzed using HPLC equipped with a Hyper REZ XP
2.2.5. Temperature-programmed reduction
analysis. carbohydrate Ca2+ 8 mm column and an ERC RefractoMax 520
Temperature-programmed reduction of hydrogen was carried detector. The diols were analyzed on a SP-6890 gas chromato-
out in a quartz U-tube reactor, and 300 mg sample was used for graph equipped with a SE-30 column (0.25 mm ꢂ 50 m) and a
Table 3 Catalytic performance of different catalysts in the hydrogenolysis of xylitol
Yield (%)
Conversion
Catalysts
(%)
EG
1,2-PG
1,3-PG
Ytotal (%)
2% nano-Ni/meso-Ce–TiO2
4% nano-Ni/meso-Ce–TiO2
6% nano-Ni/meso-Ce–TiO2
8% nano-Ni/meso-Ce–TiO2
8% Ni/meso-Ce–TiO2 (IMP)
42.1
61.3
77.3
87.9
74.5
21.8
34.7
45.5
50.2
41.4
6.1
10.9
11.8
17.0
14.9
2.8
6.5
12.9
16.0
11.9
30.7
52.1
70.2
83.2
68.2
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ame ionization detector. The catalytic stability was investi- crystallite size of the Ni nanoparticles would be reduced by the
gated by ltration without any treatment in the next run.
introduction of a small amount of cerium species.
The liquid N2 adsorption–desorption isotherms and the pore
size distribution for the 8% nano-Ni/meso-Ce–TiO2 sample
under different calcination temperatures are shown in Fig. 5,
and the physical properties are listed in Table 2. All the samples
3. Results and discussion
3.1. Sample characterization
Fig. 2 shows the XRD patterns of the meso-TiO2 and nano-Ni/ can keep their mesoporous character well, and their pore size is
meso-TiO2 samples with different Ni nanoparticle loadings. uniform. From Table 2, a pore size from 6.2 nm to 4.8 nm and a
From Fig. 2, it can be seen that the meso-TiO2 sample shows a specic surface area from 126.6 m2 gꢀ1 to 137.5 m2 gꢀ1 can be
typical anatase tetragonal crystal structure and its crystallite obtained when the calcination temperature increases from 623
size is about 7.5 nm calculated by the Scherrer equation,28 K to 673 K, which may be due to the stronger interaction
which is much smaller than that of commercial TiO2 samples. between the Ni nanoparticles and the meso-Ce–TiO2 carrier
For different nano-Ni/meso-TiO2 samples, the characteristic resulting in the increase of the Ni nanoparticle dispersity.33
diffraction peak corresponding to the Ni (111) phases29 is However, with a further increase of the calcination temperature,
present in the all samples, whose intensity increases with the the surface area decreased, which would be attributed to the
increasing Ni nanoparticle loading. For the 8% nano-Ni/meso- sintering of TiO2 probably.34 Therefore, the appropriate calci-
TiO2 sample, the crystallite size of the TiO2 and Ni nanoparticles nation temperature for the 8% nano-Ni/meso-Ce–TiO2 sample
is about 8.5 nm and 19.4 nm calculated by the Scherrer equa- is 673 K.
tion, respectively.
In order to understand the nature of the Ni nanoparticles
In order to reduce the crystallite size of the Ni nanoparticles, clearly, the H2-TPR, SEM, TEM and FT-IR techniques were
a cerium species was introduced into the nano-Ni/meso-TiO2 utilized. The H2-TPR proles of different samples are shown in
samples.30,31 XRD patterns, N2 adsorption–desorption and pore Fig. 6. One major peak between 627 K and 716 K was observed
size distributions of the meso-Ce–TiO2 and nano-Ni/meso-Ce– for samples a–f prepared by the one-pot method, which was
TiO2 samples are shown in Fig. 3. Compared to the 8% nano-Ni/ assigned to the reduction of NiO interacting with the carrier.35,36
meso-TiO2 sample, it can be seen from Fig. 3(A) that the char- Compared to sample a, the peak for sample b was shied to a
acteristic diffraction peak corresponding to the Ni (111) phases higher temperature because of the stronger interaction, which
for the 8% nano-Ni/meso-Ce–TiO2 sample became wider and implied that the introduction of a small amount of cerium
could be ignored owing to the smaller crystallite size of the Ni species can increase the dispersion of the Ni nanoparticles.37 At
nanoparticles, which is about 8.5 nm calculated by the Scherrer the same time, the interaction was strengthened with the
equation. A strong peak around 1ꢁ from Fig. 3(B) and typical calcination temperature increased. Two major peaks, one at 553
type-IV N2 adsorption–desorption isotherms from Fig. 3(C) can K and another at 818 K, were observed for sample g prepared by
be observed for all the samples, suggesting they have a hexag- the impregnation method, which is due to the reduction of bulk
onal ordered mesoporous structure.29 With the increasing Ni NiO and NiO interacting with the carrier, respectively. It can be
nanoparticle loading, the peak would be shied to a higher concluded that the Ni nanoparticles are not uniformly
degree, which indicates that the interplanar spacing decreased dispersed by the impregnation method.38
on the basis of the Bragg equation.32 The physical properties for
The SEM photos and election mapping of particles for the
all the samples are listed in Table 1. It can be seen that the 8% nano-Ni/meso-Ce–TiO2 sample are shown in Fig. 7. As dis-
samples have a large specic surface area, pore volume and pore played in Fig. 7(a), a granular morphology was obtained. It can
size, which are benecial to its catalytic performance, although be seen from Fig. 7(b–d) that the EDX element mapping showed
there is a decrease with the increase of the Ni nanoparticle uniform X-ray intensities of the Ni and Ce signals, suggesting a
loading.
The calcination temperature would affect severely the inter- Ce–TiO2 matrix,39 which agrees with the XRD analysis.
action of the components of the samples.33 The inuence of
Fig. 8 shows the TEM images for the 8% nano-Ni/meso-Ce–
homogeneous distribution of the Ni nanoparticles in the meso-
calcination temperature on the structure of 8% nano-Ni/meso- TiO2 sample calcined at 673 K. From Fig. 8(A), the typical
Ce–TiO2 was studied, and the XRD patterns are shown in Fig. 4. ordered mesostructure was obviously observed, which was in
When the calcination temperature increased up to 723 K, the good agreement with the intense XRD peak at a low-angle.
diffraction peak corresponding to Ni (111) can be obviously Additional proof for the crystallinity analysis of the framework
observed, and the peak intensity would be promoted with the walls is given by the high-resolution TEM image (Fig. 8(B)).
calcination temperature increasing, which indicates that some
The (111) planes can be identied by the typical interfringe
bulk aggregates of the Ni species were formed. The average distance of 0.20 nm, which is close to the lattice spacing of the
crystallite size of the Ni nanoparticles calculated by the Scherrer (111) planes of the face-centered cubic Ni crystal (0.203 nm). It
equation is listed in Table 2. When the calcination temperature reveals that several crystalline Ni nanoparticles with well-
was 673 K, the average crystallite size of the Ni nanoparticles dened lattice planes exist.40,41
was smaller than tht of the others, which was about 8.5 nm.
FT-IR spectra of different samples are shown in Fig. 9. The
Although the calcination temperatures are the same, compared bands between 400 and 900 cmꢀ1 could be assigned to the
to sample f, the characteristic diffraction peak corresponding to stretching vibration of the Ti–O bond.42 Compared to sample a,
Ni (111) for sample a is broader, which can conclude that the a new band at about 1300 cmꢀ1, which is attributed to the
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emergence of Ti–O–M (M ¼ metal ions), is observed for the Funded by the Priority Academic Program Development of
other three samples when the cerium or nickel species is Jiangsu Higher Education Institutions.
introduced. It can be concluded that the cerium or nickel
species can be inserted into the framework of the mesoporous
TiO2 sample.42–45
Notes and references
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a xylitol
conversion of 87.9% and a diol total yield of 83.2%. However,
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by the one-pot method is much higher than that of the 8% Ni/
meso-Ce–TiO2 (IMP) sample prepared by the impregnation
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can be seen that the total yield of diols decreased slightly, and it
can reach about 78.7% aer repeating the reaction 8 times,
which suggested that the catalytic stability is kept well.
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4. Conclusions
Ni nanoparticles well-dispersed on meso-Ce–TiO2 by one-step in
the AcHE sol–gel system have been developed in this article and
investigated in the hydrogenolysis of xylitol. Characterization
results revealed that the prepared samples possessed an
ordered mesoporous character. Meanwhile, a small amount of
cerium species introduced into the meso-TiO2 can increase the
stability of the Ni nanoparticles and reduce the crystallite size of
the Ni nanoparticles. The catalytic performance of 8% nano-Ni/
meso-Ce–TiO2 was much higher than that of 8% Ni/meso-Ce–
TiO2 (IMP) although their Ni species loading is the same. The
catalytic stability for 8% nano-Ni/meso-Ce–TiO2 is kept well,
and the total yield of diols is 78.7% aer repeating the reaction
8 times. The method can provide a guide for the preparation of
non-noble metal catalysts with high catalytic performance and
give these samples a good application prospect.
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Acknowledgements
This work was supported by Jiangsu Planned Projects for Post-
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