Controlled synthesis of Pd-NiO@SiO2 mesoporous core-shell nanoparticles and their enhanced catalytic performance for p-chloronitrobenzene hydrogenation with H2

Article abstract of DOI:10.1039/c4cy00996g

In this work, Pd-NiO@SiO₂ core-shell mesoporous nanocatalysts with ～4 nm Pd-NiO heteroaggregate nanoparticle cores and ～17 nm mesoporous silica shells were successfully synthesized by a sol-gel method. The surfactant-capped PdNi alloy nanoparticles were coated with SiO₂ through hydrolysis of tetraethylorthosilicate to obtain PdNi@SiO₂ nanoparticles, and the mesoporous Pd-NiO@SiO₂ core-shell nanocatalysts were formed after removal of surfactants by calcination at 500 °C and subsequent H₂ reduction at 200 °C. The characterization results by XRD, TEM and BET revealed that Pd-NiO@SiO₂ nanocatalysts were highly stable with the maintenance of intact core-shell structures under high-temperature thermal treatments. The Pd-NiO@SiO₂ nanocatalysts illustrated a superior catalytic performance for p-chloronitrobenzene hydrogenation with H₂ to the control Pd@SiO₂ nanocatalysts. The catalytic performance enhancement of Pd-NiO@SiO₂ nanocatalysts is ascribed to the strong interaction between Pd and NiO in the cores, where the interfaces may be beneficial for hydrogenation reactions.

Full text of DOI:10.1039/c4cy00996g

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Controlled synthesis of Pd–NiO@SiO2 mesoporous
core–shell nanoparticles and their enhanced
catalytic performance for p-chloronitrobenzene
hydrogenation with H2†
Cite this: DOI: 10.1039/c4cy00996g
b
a
b
Hongmei Liu,ab Kai Tao, Chunrong Xiong and Shenghu Zhou
*
In this work, Pd–NiO@SiO2 core–shell mesoporous nanocatalysts with ~4 nm Pd–NiO heteroaggregate
nanoparticle cores and ~17 nm mesoporous silica shells were successfully synthesized by a sol–gel
method. The surfactant-capped PdNi alloy nanoparticles were coated with SiO2 through hydrolysis of
tetraethylorthosilicate to obtain PdNi@SiO2 nanoparticles, and the mesoporous Pd–NiO@SiO2 core–shell
nanocatalysts were formed after removal of surfactants by calcination at 500 °C and subsequent H2
reduction at 200 °C. The characterization results by XRD, TEM and BET revealed that Pd–NiO@SiO2 nano-
catalysts were highly stable with the maintenance of intact core–shell structures under high-temperature
thermal treatments. The Pd–NiO@SiO2 nanocatalysts illustrated a superior catalytic performance for
p-chloronitrobenzene hydrogenation with H2 to the control Pd@SiO2 nanocatalysts. The catalytic perfor-
mance enhancement of Pd–NiO@SiO2 nanocatalysts is ascribed to the strong interaction between Pd and
NiO in the cores, where the interfaces may be beneficial for hydrogenation reactions.
Received 1st August 2014,
Accepted 28th August 2014
DOI: 10.1039/c4cy00996g
www.rsc.org/catalysis
surface of the active metal and allow products to diffuse out
the cores, where the reaction occurs. Typically, SiO2,17 TiO2,18
1. Introduction
As a new kind of multifunctional composite nanomaterials,
core–shell structured nanomaterials have been intensively
studied in recent decades.1–4 After coating with shell mate-
rials, not only the stability and dispersion of core materials
are significantly enhanced, but physicochemical properties
are also improved, resulting in a large number of new charac-
teristics, such as good monodispersity, regulated core–shell
ratio, stable physicochemical properties and so on.5 Due to
their unique properties, core–shell nanomaterials have broad
applications specially in the fields of energy materials,6 opto-
electronic technology,7 biology,8 and catalysis.9,10
Core–shell nanoparticles (NPs) using monometallic NPs as
cores and oxides as shells have been extensively studied in
recent years.11–16 By coating active metals with mesoporous
shells, the heterogeneous catalysts could be easily separated
and recycled. The mesoporous shells could protect the active
metal cores from sintering and allow reactants to reach the
Al2O3,19 Fe2O3,20 and SnO2 (ref. 21) are the common shell
materials. Among the abovementioned shell materials, SiO2
is the most widely adopted material due to its high thermal
stability, controllable morphology and desirable surface
area.5 The Stöber method22 and sol–gel process23 were mostly
used to encapsulate metal NPs with a mesoporous SiO2 shell.
The thickness of the mesoporous SiO2 shell can be controlled
by adjusting the pH value of the solution and the tetraethoxy-
silane (TEOS)/water ratio. The resultant mesoporous core–
shell structures after removal of templates by calcination
allow catalytic reactions to occur on the surface of the metal
cores. Yin and co-workers reported mesoporous Au@SiO2
core–shell NPs by the sol–gel method, and the corresponding
CO oxidation over the core–shell nanocatalysts was also
studied.24 In our previous work, Pd@SiO2 mesoporous nano-
catalysts were successfully prepared in a water system using
tetradecyl trimethyl ammonium bromide (TTAB) as a template
for hydrolysis of TEOS,25 and the thickness of the silica shells
can be well controlled by changing the synthetic parameters.
The obtained mesoporous Pd@SiO2 nanocatalysts after calci-
nation and following H2 treatment illustrated superior cata-
lytic and thermal stability.
a College of Materials and Chemical Engineering, Hainan University, Haikou,
Hainan 570228, PR China
b Ningbo Institute of Materials Technology and Engineering, Chinese Academy of
Sciences, Ningbo, Zhejiang 315201, PR China. E-mail: zhoush@nimte.ac.cn;
Fax: +86 574 86685043; Tel: +86 574 86696927
Bimetallic NPs illustrate enhanced catalytic performance
to their monometallic counterparts.26–28 However, in some
cases, the catalytic selectivity enhancement over bimetallic
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00996g
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NPs is obtained at the expense of catalytic activity.29 To
obtain high activity and high selectivity at the same time,
strong metal–support interaction catalysts have been devel-
demonstrates the synthetic procedure for Pd–NiO@SiO2
mesoporous nanocatalysts.
oped for various catalytic reactions. Corma and co-workers 2. Experimental section
reported high-temperature hydrogen-reduced Pt/TiO2 cata-
2.1. Chemicals
lysts for hydrogenation of substituted nitrobenzenes.30 The
Potassium tetrachloropalladate(II) (K2PdCl4), nickel(II)
high-temperature H2 treatment causes the Pt NPs to be
covered by a thin layer of partially reduced TiOx, and the
strong interaction between Pt and TiOx stabilized Pt NPs and
produced a highly active interface for catalytic hydrogenation.
Zhou et al.31,32 developed highly active Au–NiO/SiO2 catalysts
for low-temperature CO oxidation, where the strong interac-
tion was generated by in situ transformation of bimetallic
AuNi alloy NPs to close contact Au–NiO heteroaggregate NPs.
Similarly, enhanced catalytic performance of Pt–MxOy/Al2O3
(M = Ni, Fe, Co) heteroaggregate nanocatalysts by transforma-
tion of PtM alloy NPs was also reported.33
acetylacetonate (Ni(acac)2, 95%), sodium borohydride
(NaBH4, 98%), ammonium hydroxide (NH4OH, AR), tetra-
ethylorthosilicate (TEOS, AR), and tetradecyl trimethyl ammo-
nium bromide (TTAB, AR) were purchased from Aladdin.
p-Chloronitrobenzene (p-CNB, AR) was purchased from
Shanghai Chemical Reagent Company. Deionized water was
purchased from local sources. All reagents were used without
further purification.
2.2. Synthesis
Here, we reported a successful synthesis of Pd–NiO@SiO2
mesoporous nanocatalysts, where the strong metal–oxide
interaction and mesoporous core–shell structures coexist.
Catalytic hydrogenation of p-chloronitrobenzene with
H2 was used to investigate the catalytic performance of
Pd–NiO@SiO2. In this work, TTAB-capped PdNi alloy NPs
were synthesized by co-reduction of Pd and Ni precursors
using NaBH4 as a reducing agent. SiO2 shells were then used
to coat the PdNi alloy NPs to obtain PdNi@SiO2 core–shell
NPs through the hydrolysis of TEOS. The mesoporous
Pd–NiO@SiO2 or PdNi@SiO2 nanocatalysts were obtained
after calcination and following H2 reduction under different
conditions. The Pd–NiO@SiO2 nanocatalysts illustrated a
superior catalytic performance for catalytic p-chloronitrobenzene
(p-CNB) hydrogenation to the control Pd@SiO2 mesoporous
nanocatalysts synthesized by a similar method, and their
reusability was also studied. The enhanced catalytic perfor-
mance over Pd–NiO@SiO2 is ascribed to core–shell structures
and the strong interaction between Pd and NiO. Scheme 1
2.2.1. Synthesis of TTAB-capped PdNi alloy NPs. In a typical
synthesis, 8.7 mg of Ni(acac)2 was fully dissolved in 85 mL of
deionized water in a 250 mL three-necked round-bottomed
flask by 5 minutes of ultrasonic treatment at room tempera-
ture. An aqueous solution of K2PdCl4 (2.8 mL, 10.0 mM) was
then added into the above solution with magnetic stirring,
followed by addition of 0.840 g of TTAB. The resultant
mixture was stirred at 30 °C for ~10 minutes to fully dissolve
TTAB. Ice-cold NaBH4 solution (5.0 mL, 0.93 mM) was then
injected into the system using a syringe. The syringe was kept
in place to release the gas generated in the reaction, and then
the flask was sealed to make a closed system. The mixture
was further stirred at a low speed at 30 °C for 2.5 hours.
TTAB-capped PdNi NPs were collected by centrifugation,
washed with acetone once, and then dried in air for future
characterization.
2.2.2. Synthesis of PdNi@SiO2 core–shell NPs. PdNi@SiO2
core–shell NPs with ~17 nm thick silica shells at a TEOS/Pd
ratio of 370 : 1 were synthesized by a modified sol–gel
Scheme 1 Schematic demonstration of synthetic procedures of Pd–NiO@SiO2 and PdNi@SiO2 mesoporous core–shell nanocatalysts.
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process.25 The abovementioned as-synthesized PdNi colloids
(24.0 mL, 0.0072 mmol Pd and 0.0082 mmol Ni) were mixed
with 120 mL of deionized water in a 250 mL three-necked
round-bottomed flask at room temperature with magnetic
stirring. A certain amount of concentrated ammonia aqueous
solution (25–28 wt%) was then added to the solution to
adjust the pH value of the solution to 11.6. TEOS (600 μL,
2.69 mmol) was then added into the system, and the reaction
system was stirred for 1.5 hours at room temperature. The
gray powdery PdNi@SiO2 products were obtained by centrifu-
gation and dried at 60 °C for 10 hours for future use.
2.2.3. Synthesis of Pd–NiO@SiO2 and PdNi@SiO2 core–
shell mesoporous nanocatalysts. The above dried PdNi@SiO2
NPs were ground and then calcined in a muffle furnace at
500 °C for 3 hours to remove TTAB surfactants. The resulting
brown powders were further reduced by H2 with a gas flow
rate of 60 mL min−1 in a fixed bed reactor at 200 °C and
500 °C for 2 hours to obtain Pd–NiO@SiO2 and PdNi@SiO2
core–shell nanocatalysts, respectively.
nanocatalysts were determined by using a PE Optima 2100DV
inductively coupled plasma optical emission spectrometer
(ICP-OES). H2 temperature-programmed reduction (H2-TPR)
was carried out using a FINESORB3010 instrument with a
thermal conductivity detector (TCD). The infrared (IR) spectra
of the nanocatalysts were recorded in transmission mode
using a Bruker Tensor 27 spectrophotometer. The thermal
property of the samples was measured by using a Pyris Dia-
mond thermogravimetric analyzer (TGA). During the test, the
samples were heated from 50 °C to 500 °C at a heating rate
of 5 °C min−1 in an air atmosphere with a gas flow rate of
50 mL min−1. Brunauer–Emmett–Teller (BET) surface area,
pore size distribution and the adsorption/desorption iso-
therms of the nanocatalysts were measured by N2 adsorption
at 77 K, using a Micromeritics ASAP-2020 M automatic spe-
cific surface area and porous physical adsorption analyzer.
2.4. Catalytic hydrogenation of p-chloronitrobenzene with H2
In this study, the hydrogenation of p-CNB with H2 at ambient
pressure was carried out to investigate the catalytic perfor-
mance of different Pd-containing core–shell nanocatalysts
with ~17 nm thick SiO2 shells. Pd–NiO@SiO2 mesoporous
nanocatalysts (0.19 g) with actual Pd loading of 0.36 wt% and
1.0 g of p-CNB were mixed with 24 mL of absolute ethanol in
a 50 mL three-necked round-bottomed flask at room temper-
ature. The system was purged with high-purity H2 to remove
air prior to reaction. The mixture was then heated to 30 °C
and maintained at 30 °C for a certain reaction time under
hydrogen flow with vigorous magnetic stirring. The liquid
products were collected by centrifugation to remove the solid
catalysts and were analyzed online using a gas chromato-
graph equipped with a flame ionization detector (FID). For
control experiments, 0.10 g of Pd@SiO2 mesoporous nano-
catalysts with actual Pd loading of 0.68 wt% was used to
maintain the same amount of Pd as that of Pd–NiO@SiO2
nanocatalysts, and other conditions were the same as those
of Pd–NiO@SiO2 catalytic experiments.
To study the durability of Pd–NiO@SiO2 nanocatalysts,
cycle-to-cycle experiments were carried out. Pd–NiO@SiO2
nanocatalysts (0.19 g) with an actual Pd loading of 0.36 wt%
and 1.0 g of p-CNB were mixed in 24 mL of absolute ethanol
in the first cycle. The catalytic reactions were carried out at
30 °C and ambient H2 pressure. After each reaction run, the
solid nanocatalysts were collected by centrifugation, washed
twice with absolute ethanol, and dried in oven at 60 °C for
the next reaction cycle. Because of the loss of catalysts during
the recovery process, lesser amounts of nitrobenzene and
ethanol in the next reaction cycle were used to keep the same
ratio of p-CNB/catalysts and p-CNB/solvent as those of the
first cycle. Due to the loss of catalysts, 5 cycles of catalytic
reactions were carried out.
2.2.4. Synthesis of control Pd@SiO2 nanocatalysts.
Pd@SiO2 catalysts with ~17 nm thick silica shells at a TEOS/
Pd ratio of 230 : 1 were prepared according to the literature.25
Briefly, K2PdCl4 aqueous solution (7.5 mL, 10.0 mM), TTAB
(39.0 ml, 100 mM) aqueous solution and 90 mL of deionized
water were mixed in a 250 mL three-necked round-bottomed
flask. The system was stirred for ~20 minutes. Ice-cold NaBH4
(9.0 mL, 250 mM) aqueous solution was then injected into
the system using a syringe. The syringe was kept in place to
release the gas generated in the reaction, and then the flask
was sealed to make a closed system. The obtained reaction
mixture was further stirred for 15 hours at a low speed to
obtain Pd colloids with an average Pd particle size of 4.2 nm.
To coat the Pd cores with silica shells, the abovementioned
as-synthesized Pd colloids (24.0 mL, 0.0125 mmol Pd) were
mixed with 120 mL of deionized water in a 250 mL three-
necked round-bottomed flask at room temperature with
magnetic stirring, and a few drops of concentrated ammonia
solution (25–28 wt%) were then added to adjust the pH of
the solution to 10.6. TEOS (642 μL, 2.88 mmol) was then
introduced into the solution after 10 minutes of stirring. The
resultant mixture was further stirred for 1 hour at room tem-
perature. The collection procedure and treatment process of
Pd@SiO2 core–shell nanocatalysts were the same as those of
Pd–NiO@SiO2.
2.3. Characterization
X-ray diffraction (XRD) measurements were carried out using
a Bruker D8 Advance X-ray diffractometer with Cu Kα radia-
tion in the 2θ range from 10° to 80°. Transmission electron
microscopy (TEM) images were obtained using a JEOL 2100
transmission electron microscope. X-ray photoelectron spec-
troscopy (XPS) spectra were acquired by using an AXIS
ULTRADLD multifunctional X-ray photoelectron spectroscope
with an Al source. The data processing was performed by the
CasaXPS software. The actual metal loadings of the
3. Results and discussion
In this study, a PdNi alloy colloid was synthesized through
co-reduction of Ni(acac)2 and K2PdCl4 by NaBH4 using TTAB
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as a capping agent. The molar ratio of Ni(acac)2/K2PdCl4 was
set to 1.15 : 1.00 to ensure the formation of 1 : 1 PdNi alloy
NPs by XRD measurement. SiO2 shells were then used to coat
the PdNi alloy cores to form PdNi@SiO2 NPs by a sol–gel
process through the hydrolysis of TEOS, and the thickness of
SiO2 shells was controlled by adjusting the pH value of the
solution and the TEOS/Pd ratios. The TTAB surfactants on
PdNi alloy surfaces were removed by calcination at 500 °C,
and the obtained PdO–NiO@SiO2 NPs were then reduced by
H2 reduction at 200 °C and 500 °C to obtain Pd–NiO@SiO2
and PdNi@SiO2 nanocatalysts, respectively. For comparison
of catalytic activity, the Pd@SiO2 nanocatalysts with the same
SiO2 shell thickness were also prepared by a similar process.
The actual Pd and Ni loadings of corresponding nanocatalysts
were determined by ICP-OES.
TEM images of various NPs are shown in Fig. 1. As shown
in Fig. 1a, TTAB-capped PdNi NPs demonstrated irregular
shapes with an average particle size of 4.2 nm, and a lattice
spacing of 0.217 nm in the HRTEM image was clearly visible,
which is in line with the lattice spacing of an alloyed PdNi
(111) plane. After encapsulation of PdNi NPs with SiO2 shells,
the PdNi cores and the SiO2 shells were clearly observed in
PdNi@SiO2 NPs as shown in Fig. 1b–f. The synthetic condi-
tions were found to have a profound effect on the structures
of PdNi@SiO2 NPs, especially the pH value. When the pH
values were adjusted to 11.5, 11.6, 11.7 and 11.8, the corre-
sponding thicknesses of the SiO2 shells in PdNi@SiO2
NPs were ~14 nm, ~17 nm, ~23 nm and ~28 nm, respectively.
The correlation between pH value and the SiO2 thickness
suggested that slow hydrolysis of TEOS at a pH value of 11.5
resulted in small NPs with some percentage of NPs without
PdNi cores, while fast hydrolysis at 11.8 resulted in large NPs
with several PdNi NPs encapsulated into one silica particle,
which is consistent with the finding in the synthesis of
Pd@SiO2 NPs.25
The thickness of the SiO2 shell of PdNi@SiO2 NPs was
controlled by the TEOS/Pd ratio. As shown in Fig. 1c and f,
when the molar ratio of TEOS/Pd increased from 370 : 1 to
490 : 1, the thickness of the SiO2 shells increased from
~17 nm to ~20 nm at the same pH value of 11.6. In this
study, the synthetic conditions under a pH value of 11.6 and
a TEOS/Pd ratio of 370 : 1 are optimum since PdNi@SiO2 NPs
synthesized under these conditions have perfect core–shell
structures and thin SiO2 shells. Therefore, the PdNi@SiO2
NPs prepared under these conditions will be treated under
different conditions and tested for catalytic hydrogenation of
p-CNB with H2. The size distributions of PdNi NPs and PdNi
cores in PdNi@SiO2 NPs are illustrated in Fig. 2a and b. It
was clearly demonstrated that PdNi cores maintained the
same size as that of as-prepared PdNi bimetallic NPs.
XRD patterns of PdNi bimetallic NPs and as-prepared
PdNi@SiO2 NPs with a SiO2 shell thickness of ~17 nm are
shown in Fig. 3. As shown in Fig. 3a, the PdNi bimetallic NPs
showed a broad diffraction at 2θ of 41.6° without any diffrac-
tions of monometallic Pd and Ni, which is consistent with
the diffraction of (111) planes of PdNi alloy and the lattice
spacing of 0.217 nm in the inset of Fig. 1a. The weak XRD
diffractions of PdNi alloy NPs and lack of diffractions at high
2θ values were consistent with their small sizes around 4.2 nm.
After coating PdNi alloy NPs with SiO2, the diffractions of
PdNi alloy NPs (Fig. 3b) became invisible due to the low
weight percentage of PdNi in PdNi@SiO2 NPs, and only
broad XRD diffraction of amorphous SiO2 at 2θ of 21.9° was
present.
To obtain the oxidation state of PdNi alloy nanoparticles,
XPS studies were carried out. As shown in Fig. 4a, four peaks
Fig. 1 TEM images of different NPs showing a) TTAB-capped PdNi NPs (the inset is an HRTEM image of one nanoparticle showing a lattice spacing
of 0.217 nm); b) PdNi@SiO2 NPs synthesized at a pH value of 11.5 and a TEOS/Pd ratio of 370 : 1; c) PdNi@SiO2 NPs synthesized at a pH value
of 11.6 and a TEOS/Pd ratio of 370 : 1; d) PdNi@SiO2 NPs synthesized at a pH value of 11.7 and a TEOS/Pd ratio of 370 : 1; e) PdNi@SiO2 NPs synthe-
sized at a pH value of 11.8 and a TEOS/Pd ratio of 370 : 1; f) PdNi@SiO2 NPs synthesized at a pH value of 11.6 and a TEOS/Pd ratio of 490 : 1;
g) Pd–NiO@SiO2 nanocatalysts obtained after 500 °C calcination and following 200 °C H2 reduction of PdNi@SiO2 synthesized at a pH value of
11.6 and a TEOS/Pd ratio of 370 : 1; h) PdNi@SiO2 nanocatalysts obtained after 500 °C calcination and following 500 °C H2 reduction of PdNi@SiO2
synthesized at a pH value of 11.6 and a TEOS/Pd ratio of 370 : 1. Scale bar of a)—20 nm; scale bar of the inset in a)—2 nm; scale bars of b)–h)—50 nm;
scale bar of the inset in g)—20 nm.
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Fig. 2 Size distribution of Pd-containing particles in different systems showing a) PdNi NPs capped with TTAB; b) PdNi@SiO2 NPs synthesized at a
pH value of 11.6 and a TEOS/Pd ratio of 370 : 1; c) Pd–NiO@SiO2 nanocatalysts obtained after 500 °C calcination and following 200 °C H2 reduc-
tion; d) PdNi@SiO2 nanocatalysts obtained after 500 °C calcination and following 500 °C H2 reduction.
with binding energies of 335.1, 336.1, 340.4 and 341.7 eV
were observed. According to the literature,34,35 the binding
energies of 335.1 and 340.4 eV were ascribed to 3d5/2 and
3d3/2 of zero-valent Pd, while the binding energies of 336.1
and 341.7 eV were consistent with the binding energies of
3d5/2 and 3d3/2 of oxidized Pd2+, respectively. The appearance
of Pd2+ species is due to the oxidation of surface Pd atoms by
O2 in the air, and a Pd2+/Pd0 ratio of 29.1 : 70.9 by XPS curve
fitting36 was observed. The XPS spectrum of Ni is shown
in Fig. 4b. The binding energies of 855.3, 860.9, 872.9 and
878.9 eV were assigned to 2p3/2, 2p3/2 satellite, 2p1/2, and
2p1/2 satellite of Ni2+ 37–39
respectively. A very weak peak with
,
a binding energy of 851.7 eV was also observed and can be
assigned to 2p3/2 of zero-valent Ni species. It is well known
that Ni is easily oxidized in air, and the literature has
reported that Ni metallic NPs are fully transformed into NiO
NPs after their exposure to air.40 Since XPS is a surface-
sensitive technology and surface Ni atoms are more easily
oxidized, the dominant Ni2+ species in PdNi alloy NPs by XPS
measurement is reasonable.
For catalytic applications, the remaining TTAB in
PdNi@SiO2 NPs should be removed by calcination to open
the mesoporous channels of the SiO2 shell so that reactants
can contact with the reactive metal cores and the products
can diffuse out through the mesoporous channels. Therefore,
the thermal treatments of PdNi@SiO2 NPs were investigated.
The TG/DTA profiles of the as-synthesized PdNi@SiO2 NPs
with a shell thickness of ~17 nm are illustrated in the left
panel of Fig. 5. The TTAB surfactants began to be removed at
150 °C, and the complete removal of TTAB was observed
around 400 °C. The FT-IR spectra of PdNi@SiO2 with a shell
Fig. 3 XRD patterns of NPs showing a) as-prepared PdNi alloy NPs;
b) as-prepared PdNi@SiO2 NPs; c) PdO–NiO@SiO2 NPs obtained by
500 °C calcination of PdNi@SiO2 NPs; d) Pd–NiO@SiO2 nanocatalysts
obtained by 500 °C calcination and following 200 °C reduction of
PdNi@SiO2 NPs; e) PdNi@SiO2 nanocatalysts obtained by 500 °C
calcination and following 500 °C reduction of PdNi@SiO2 NPs. All
PdNi@SiO2 samples before treatments were synthesized under the
conditions of a pH value of 11.6 and a TEOS/Pd ratio of 370 : 1. Vertical
lines indicate PdNi (JCPDS 03-065-9444), Ni (JCPDS 01-089-7129),
PdO (JCPDS 00-043-1024), and Pd (JCPDS 01-071-3757).
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Fig. 4 XPS spectra of PdNi alloy showing a) Pd and b) Ni.
Fig. 5 TG/DTA curves of as-synthesized PdNi@SiO2 NPs (left panel) and FT-IR spectra of PdNi@SiO2 NPs before and after 500 °C calcination (right panel).
thickness of ~17 nm before and after 500 °C calcination are
shown in the right panel of Fig. 5. The absorption peaks at
2926 cm−1 and 2852 cm−1 before calcination were ascribed to
antisymmetric and symmetric stretching modes of methylene
of TTAB,41,42 respectively. However, the absorption peaks of
methylene disappeared after calcination at 500 °C, indicating
the complete removal of TTAB at 500 °C.
the presence of weak Pd (111) diffractions at 2θ of 40.1°. The
corresponding TEM images of Pd–NiO@SiO2 nanocatalysts
are shown in Fig. 1g. It was clearly observed that the core–shell
structure remained intact after calcination at 500 °C and
following H2 reduction at 200 °C, and the inset clearly demon-
strated a porous structure of Pd–NiO@SiO2 NPs. The XRD
patterns and TEM images of PdNi@SiO2 nanocatalysts are
illustrated in Fig. 3e and 1h, respectively. The absence of indi-
vidual Pd and individual Ni diffractions in Fig. 3e suggested
the presence of bimetallic PdNi NP cores in PdNi@SiO2
The XRD diffractions (Fig. 3c) at 2θ of 33.8° of PdNi@SiO2
NPs after calcination at 500 °C confirmed the formation of
PdO. The NiO diffraction is absent due to the formation of
amorphous NiO or the low weight percentage of NiO, and
this phenomenon has been recorded in the Au–NiO/SiO2
system.32 The presence of Ni in the PdO–NiO@SiO2 NPs
obtained by calcination of PdNi@SiO2 NPs was evidenced
by ICP-OES measurement. Since the active component
for catalytic hydrogenation is zero-valent Pd species, the
PdO–NiO@SiO2 NPs were subjected to H2 reaction to reduce
PdO. Fig. 6 illustrates the H2-TPR curve of PdO–NiO@SiO2
NPs. The two H2 consumption peaks at 106 °C and 123 °C
were ascribed to the reduction of PdO, while the broad peak
around 300–500 °C was ascribed to the reduction of NiO.43,44
According to the H2-TPR result of PdO–NiO@SiO2 NPs in
Fig. 6, the PdO–NiO@SiO2 NPs were reduced by H2 at 200 °C
and 500 °C to obtain Pd–NiO@SiO2 and bimetallic PdNi@SiO2
nanocatalysts, respectively. The XRD patterns of Pd–NiO@SiO2
nanocatalysts in Fig. 3d confirmed the formation of Pd by
Fig. 6 H2-TPR curves of PdO–NiO@SiO2 NPs.
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Table 2 The real loading of Pd and Ni in Pd-containing core–shell
mesoporous nanocatalysts by ICP-OES
nanocatalysts, and the weak diffractions at 2θ of 40.8° is due
to the formation of PdNi alloy NPs with concentration gradi-
ents inside the NPs possibly by surface segregation. The corre-
sponding TEM images in Fig. 1h illustrated porous intact
core–shell structures, showing the high thermal stability of
PdNi@SiO2 nanocatalysts. Moreover, the size analysis of cores
of Pd–NiO@SiO2 (Fig. 2c) and PdNi@SiO2 (Fig. 2d) nano-
catalysts revealed nearly the same average size before and after
thermal treatment, further confirming the high thermal stabil-
ity of the materials.
Sample
Pd loading (wt%)
Ni loading (wt%)
Pd@SiO2
Pd–NiO@SiO2
PdNi@SiO2
0.68
0.36
0.36
—
0.23
0.24
and PdNi@SiO2 nanocatalysts are 0.68 wt%, 0.36 wt%,
and 0.36 wt%, respectively, and the molar ratios of Pd/Ni
in the Pd–NiO@SiO2 and PdNi@SiO2 nanocatalysts are close
to 1 : 1.
The porosity of PdO–NiO@SiO2 NPs, Pd–NiO@SiO2 and
PdNi@SiO2 nanocatalysts was examined by N2 adsorption
experiments. Adsorption–desorption isotherms and the pore
size distribution are shown in Fig. 7. The BET surface areas,
pore volumes and pore sizes of different samples are summa-
rized in Table 1. Typical type IV isotherms in Fig. 7 according
to IUPAC nomenclature45 were observed for all samples, indi-
cating the formation of mesopores by removal of TTAB sur-
factants through calcination at 500 °C. The mesopores with
a mean pore size of around 2.2 nm were found. The BET
surface areas of PdO–NiO@SiO2 NPs, Pd–NiO@SiO2 and
The hydrogenation of p-chloronitrobenzene (p-CNB) to
p-chloroaniline (p-CAN) is accompanied by side reactions
such as dechlorination to aniline (AN). The supported Pd
catalysts exhibited moderate p-CAN selectivity,46 and the
dechlorination over Pd catalysts was more severe than that of
Pt-based catalysts.47 The literature has also reported that the
application of supported Pd catalysts was very active in
dechlorination reactions, where the dechlorination products
are desired.48 In this study, to investigate the influence of
elemental compositions of cores on dechlorination reactions,
the Pd–NiO@SiO2, PdNi@SiO2 and control Pd@SiO2 meso-
porous nanocatalysts were tested for catalytic hydrogenation
of p-chloronitrobenzene with H2 at 30 °C and ambient H2
pressure. To ensure the same mole number of Pd in each
nanocatalyst for catalytic performance comparison, different
amounts of Pd-containing core–shell nanocatalysts with the
same thickness of SiO2 shells were used since the Pd load-
ings in different catalysts were different. Table 3 summarizes
the catalytic performance of Pd–NiO@SiO2, PdNi@SiO2 and
Pd@SiO2 nanocatalysts. Indeed, the control Pd@SiO2 meso-
porous nanocatalysts with a SiO2 shell thickness of ~17 nm
showed a p-CNB conversion of 98.7% with a poor p-chloroaniline
(p-CAN) selectivity of 62.9%, showing a significant formation
of the by-product aniline produced by dechlorination reac-
tions. The p-aminophenol (p-AP) formation was also found in
this reaction. The literature has reported the formation of
p-AP during catalytic hydrogenation of nitrobenzene with
H2,49,50 and p-AP in this study could be originated from the
hydrogenation of nitrobenzene produced by dechlorination
of p-CNB and/or the dechlorination of p-CAN. In contrast, the
PdNi@SiO2 illustrated an enhanced p-CAN selectivity of
80.9% with a decreased conversion of 68.5%. The Pd–NiO@SiO2
nanocatalysts demonstrated a superior catalytic performance
to Pd@SiO2 and PdNi@SiO2 nanocatalysts. At the conversion
of 100%, the selectivity of p-CAN over Pd–NiO@SiO2 is 82.7%,
which is significantly higher than that of control Pd@SiO2
nanocatalysts.
PdNi@SiO2 nanocatalysts were around 771–791 m2 g−1
,
suggesting that the following H2 treatment at 200 °C and
500 °C did not significantly change the surface area and that
this kind of core–shell material was highly thermally stable.
The elemental compositions of Pd–NiO@SiO2, PdNi@SiO2
and the control Pd@SiO2 nanocatalysts were analyzed by
ICP-OES. Table 2 summarizes the real loadings of Pd and Ni
in Pd-containing core–shell nanocatalysts. The control Pd@SiO2
mesoporous nanocatalysts were synthesized according to the
literature,25 and the detailed characterization is presented in
the ESI.† The real Pd loadings of Pd@SiO2, Pd–NiO@SiO2
Fig. 7 N2 adsorption–desorption isotherms and pore size distribution
(inset) showing a) PdO–NiO@SiO2 NPs, b) Pd–NiO@SiO2 nanocatalysts,
and c) PdNi@SiO2 nanocatalysts.
Table 1 BET surface areas, pore volumes, and pore sizes of PdO–NiO@SiO2, Pd–NiO@SiO2 and PdNi@SiO2 nanocatalysts
Samples
BET surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Pore size (nm)
PdO–NiO@SiO2
Pd–NiO@SiO2
PdNi@SiO2
771
791
773
1.1
1.1
1.1
2.2
2.2
2.2
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Table 3 Product distribution in hydrogenation of p-CNB with H2 cata-
lyzed by Pd@SiO2, Pd–NiO@SiO2 and PdNi@SiO2 nanocatalysts with a
SiO2 shell thickness of ~17 nm
Selectivity (%)
Catalyst
Conversion (%)
p-CAN
AN
p-AP
Others
Pd@SiO2
PdNi@SiO2
Pd–NiO@SiO2
98.7
68.5
100.0
62.9
80.9
82.7
22.8
5.4
12.5
14.3
13.6
4.8
0.0
0.1
0.0
Reaction conditions: reaction temperature—30 °C; ambient H2
pressure; p-CNB—1.0 g; reaction time—2.5 hours; ethanol—24 mL;
Pd@SiO2—0.100 g (Pd loading—0.68 wt%); PdNiO@SiO2—0.190 g
(Pd loading—0.36 wt%); PdNi@SiO2—0.190 g (Pd loading—0.36 wt%);
speed of agitation—500 rpm.
Fig. 9 Effect of temperature on p-CNB conversion and selectivity of
different products for catalytic p-CNB hydrogenation over Pd–NiO@SiO2
nanocatalysts. Reaction conditions: ambient H2 pressure; Pd–NiO@SiO2
catalysts—0.190 g; p-CNB—1.0 g; 24 mL of ethanol; reaction time—
2.5 hours; speed of agitation—500 rpm.
Supported Pd catalysts were active for hydrogenation reac-
tions51 but showed moderate p-CAN selectivity for catalytic
p-CNB hydrogenation. By alloying with Ni, the p-CAN selectiv-
ity over PdNi@SiO2 is enhanced at the expense of activity,
which is due to dilution of Pd by Ni on the surface. The same
phenomenon had been well stated in the literature.30 How-
ever, the transformation of PdNi alloy NPs to close contact
Pd–NiO heteroaggregate NPs via proper treatment could
induce the strong Pd–NiO interaction, which is possibly bene-
ficial for hydrogenation reactions as suggested in the
literature.30–33 In this study, the selectivity enhancement of
p-CAN over Pd–NiO@SiO2 is ascribed to the strong Pd–NiO
interaction, and the Pd–NiO interface facilitates hydrogena-
tion reactions.
The influence of reaction time on catalytic performance
over Pd@SiO2 and Pd–NiO@SiO2 nanocatalysts at 30 °C
under ambient H2 pressure was examined, and the result is
shown in Fig. 8. The conversion of p-CNB increased from
44% at 0.5 hours to 100% at 2.5 hours for both Pd@SiO2 and
Pd–NiO@SiO2 nanocatalysts and remained 100% with further
increasing reaction time. The p-CAN selectivity of Pd–NiO@SiO2
was slightly higher than that of Pd@SiO2 at the beginning
of the reactions, but the p-CAN selectivity gap between
Pd–NiO@SiO2 and Pd@SiO2 nanocatalysts became significant
when the reaction time was increased. The influence of reaction
temperatures on the catalytic performance over Pd–NiO@SiO2
nanocatalysts is shown in Fig. 9. It was found that the conver-
sion of p-CNB increased from 86% at 20 °C to 100% at 30 °C.
On the contrary, the selectivity of p-CAN firstly increased
from 65% at 20 °C to 82% at 30 °C and then decreased to
60% at 60 °C. The result suggested that when the tempera-
ture exceeded 30 °C, the formation of a by-product is much
severe, resulting in decreased p-CAN selectivity.
Cycle-to-cycle catalytic reactions were carried out to inves-
tigate the catalytic stability of Pd–NiO@SiO2 and Pd@SiO2
nanocatalysts, and the results are listed in Table 4. Because
of the loss of catalysts during the recovery process after each
reaction cycle, the amounts of p-CNB and ethanol in the next
cycle were decreased accordingly to maintain the ratios of
p-CNB/catalysts and p-CNB/solvent identical in each reaction
run. In these cycle-to-cycle reactions, the conversion of p-CNB
for Pd@SiO2 and Pd–NiO@SiO2 was basically maintained
and approached 95.9% and 99.4% after five cycles of reac-
tions, respectively. The p-CAN selectivity over Pd–NiO@SiO2
nanocatalysts in each cycle was higher than that of Pd@SiO2
nanocatalysts and was stabilized around 66.9% after five cycles
of reactions. The average p-CAN selectivity over Pd–NiO@SiO2
Fig. 8 Effect of reaction time on p-CNB conversion (left panel) and selectivity of p-CAN, AN and p-AP (right panel) for catalytic p-CNB hydroge-
nation over Pd@SiO2 and Pd–NiO@SiO2 nanocatalysts. Reaction conditions: reaction temperature—30 °C; ambient H2 pressure; Pd@SiO2—0.100 g;
Pd–NiO@SiO2—0.190 g; p-CNB—1.0 g; 24 mL of ethanol; speed of agitation—500 rpm.
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Table 4 Cycle-to-cycle reactions of hydrogenation of p-CNB with H2 catalyzed by Pd–NiO@SiO2 and Pd@SiO2 nanocatalysts
Pd–NiO@SiO2 catalyst
Pd@SiO2 catalyst
Cycle
index
Weight (g) p-CNB (g) Conversion (%) p-CAN selectivity (%) Weight (g) p-CNB (g) Conversion (%) p-CAN selectivity (%)
1
2
3
4
5
6
0.190
0.170
0.151
0.129
0.106
0.085
1.000
0.885
0.800
0.681
0.557
0.447
100.0
99.9
99.4
98.9
97.6
99.4
82.7
68.5
67.5
69.4
68.6
66.9
0.100
0.086
0.072
0.057
0.045
0.032
1.000
0.858
0.722
0.574
0.447
0.319
98.7
97.8
98.5
96.4
98.4
95.9
62.9
61.7
59.6
61.5
60.7
59.7
Reaction conditions: reaction temperature—30 °C; p-CNB—1.0 g; reaction time—2.5 hours; atmospheric H2 pressure; speed of agitation—
500 rpm; 24 mL of ethanol was used as solvent in the first cycle, and the ratio of ethanol/p-CNB in the following cycles was kept the same as
that of the first cycle.
nanocatalysts during five cycles of reactions was 72.6%,
which is significantly higher than that over Pd@SiO2 nano-
catalysts (61.0%), illustrating the catalytic selectivity enhance-
ment over Pd–NiO@SiO2 nanocatalysts. The decrease of
p-CAN selectivity over Pd–NiO@SiO2 catalysts might be caused
by a slight dissolution of NiO since the reaction system is
acidic due to the dechlorination side reaction. Although the
selectivity decreased from the first run to the second run, the
selectivity of p-CAN was stabilized in the range of 66.9% to
69.4%, which was still higher than that of Pd@SiO2.
The activity sequence of p-CNB hydrogenation over
Pd@SiO2, PdNi@SiO2 and Pd–NiO@SiO2 is in the following
order: Pd–NiO@SiO2 > Pd@SiO2 > PdNi@SiO2. The catalytic
p-CAN selectivity sequence of p-CNB hydrogenation is in the
following order: Pd–NiO@SiO2 > PdNi@SiO2 > Pd@SiO2. In
this study, the p-CAN selectivity of PdNi@SiO2 is higher than
that of Pd@SiO2, while the activity of PdNi@SiO2 is lower
than that of Pd@SiO2, reflecting that the adjustment of
electronic state by metal–metal interaction in bimetallic alloy
NPs enhances the p-CAN selectivity but at the expense of
activity. However, both catalytic activity and selectivity
over Pd–NiO@SiO2 nanocatalysts are higher than those of
Pd@SiO2, suggesting that the interfaces between Pd and NiO
in the cores of Pd–NiO@SiO2 nanocatalysts can enhance the
catalytic activity and selectivity at the same time.
Pd and the same thickness of mesoporous silica shells. The
catalytic performance enhancement is assigned to the strong
interaction between Pd and NiO, where the interfaces
between Pd and NiO promote catalytic activity and selectivity
for p-chloronitrobenzene hydrogenation with H2. The devel-
oped Pd–NiO@SiO2 heteroaggregate nanocatalysts demon-
strated enhanced catalytic selectivity while maintaining high
catalytic activity at the same time, which is in contrast to
the selectivity enhancement at the expense of activity for
bimetallic alloy catalysts. This work provides a new method
to enhance catalytic selectivity for catalytic hydrogenation
reactions. The synthetic method developed in this study
could be extended to diverse core–shell mesoporous nano-
catalysts with metal–oxide heteroaggregate NP cores and
mesoporous shells for wide catalytic applications.
Acknowledgements
S. Zhou would like to thank the Ministry of Science and
Technology of China (grant no. 2012DFA40550) for financial
support. H. Liu would like to thank Hainan University for
financial support to the development plan of outstanding
graduate thesis.
References
4. Conclusions
1 Y. W. Lee, M. Kim, Z. H. Kim and S. W. Han, J. Am. Chem.
Soc., 2009, 131, 17036–17037.
2 L. J. Lauhon, M. S. Gudiksen, C. L. Wang and C. M. Lieber,
Nature, 2002, 420, 57–61.
3 F. Tao, M. E. Grass, Y. W. Zhang, D. R. Butcher, J. R. Renzas,
Z. Liu, J. Y. Chung, B. S. Mun, M. Salmeron and G. A. Somorjai,
Science, 2008, 322, 932–934.
4 L. Zhang, H. Jing, G. Boisvert, J. Z. He and H. Wang,
ACS Nano, 2012, 6, 3514–3527.
5 R. Ghosh Chaudhuri and S. Paria, Chem. Rev., 2012, 112,
2373–2433.
6 Y. Tak, S. J. Hong, J. S. Lee and K. Yong, J. Mater. Chem.,
2009, 19, 5945–5951.
7 T. Gao, Q. H. Li and T. H. Wang, Chem. Mater., 2005, 17,
887–892.
In this work, Pd–NiO@SiO2 and PdNi@SiO2 core–shell
mesoporous nanocatalysts were successfully synthesized by a
sol–gel process. PdNi alloy NPs were firstly synthesized by
co-reduction of K2PdCl4 and Ni(acac)2 by NaBH4 using TTAB
as a capping agent. The SiO2 shells were then used to coat
the PdNi NPs by hydrolysis of TEOS, and the thickness of
SiO2 could be tuned by adjusting the pH value of the solution
and the TEOS/Pd ratios. The resultant PdNi@SiO2 NPs were
subjected to calcination at 500 °C and subsequent H2 reduc-
tion at 200 °C or 500 °C to obtain mesoporous Pd–NiO@SiO2
or PdNi@SiO2 nanocatalysts, respectively. The Pd–NiO@SiO2
nanocatalysts illustrated a superior catalytic performance for
p-chloronitrobenzene hydrogenation with H2 to the PdNi@SiO2
and Pd@SiO2 core–shell nanocatalysts at the same amount of
This journal is © The Royal Society of Chemistry 2014
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
8 V. Salgueirino-Maceira and M. A. Correa-Duarte, Adv. Mater.,
2007, 19, 4131–4144.
9 S. Alayoglu and B. Eichhorn, J. Am. Chem. Soc., 2008, 130,
17479–17486.
10 S. Alayoglu, A. U. Nilekar, M. Mavrikakis and B. Eichhorn,
Nat. Mater., 2008, 7, 333–338.
11 T. Li, J. Moon, A. A. Morrone, J. J. Mecholsky, D. R. Talham
and J. H. Adair, Langmuir, 1999, 15, 4328–4334.
12 H. J. Cha, Y. H. Kim, Y. C. Kang and Y. S. Kang, IEEE NMDC
2006: IEEE. Nanotechnology Materials and Devices Conference
2006, Proceedings, 2006, pp. 670–671.
13 N. Zhang, S. Q. Liu, X. Z. Fu and Y. J. Xu, J. Phys. Chem. C,
2011, 115, 9136–9145.
14 X. G. Lu, G. Y. Liang, Z. B. Sun and W. Zhang, Mater. Sci.
Eng., B, 2005, 117, 147–152.
31 S. Zhou, Z. Ma, H. F. Yin, Z. L. Wu, B. Eichhorn, S. H. Overbury
and S. Dai, J. Phys. Chem. C, 2009, 113, 5758–5765.
32 S. Zhou, H. F. Yin, V. Schwartz, Z. L. Wu, D. Mullins,
B. Eichhorn, S. H. Overbury and S. Dai, ChemPhysChem,
2008, 9, 2475–2479.
33 X. D. Wang, H. B. Yu, D. Y. Hua and S. Zhou, J. Phys. Chem.
C, 2013, 117, 7294–7302.
34 E. Lundgren, G. Kresse, C. Klein, M. Borg, J. N. Andersen,
M. De Santis, Y. Gauthier, C. Konvicka, M. Schmid and
P. Varga, Phys. Rev. Lett., 2002, 88, 246103.
35 D. Zemlyanov, B. Aszalos-Kiss, E. Kleimenov, D. Teschner,
S. Zafeiratos, M. Havecker, A. Knop-Gericke, R. Schlögl,
H. Gabasch, W. Unterberger, K. Hayek and B. Klözer,
Surf. Sci., 2006, 600, 983–994.
36 J. M. Conny and C. J. Powell, Surf. Interface Anal., 2000, 29,
856–872.
37 C. P. Li, A. Proctor and D. M. Hercules, Appl. Spectrosc.,
1984, 38, 880–886.
38 G. R. Gavalas, C. Phichitkul and G. E. Voecks, J. Catal., 1984,
88, 54–64.
15 G. H. Wang and A. Harrison, J. Colloid Interface Sci., 1999,
217, 203–207.
16 N. J. Tang, L. Y. Lu, W. Zhong, C. T. Au and Y. W. Du, Sci.
China, Ser. G: Phys., Mech. Astron., 2009, 52, 31–34.
17 S. H. Joo, J. Y. Park, C. K. Tsung, Y. Yamada, P. D. Yang and
G. A. Somorjai, Nat. Mater., 2009, 8, 126–131.
18 I. Lee, J. B. Joo, Y. Yin and F. Zaera, Angew. Chem., Int. Ed.,
2011, 50, 10208–10211.
39 T. H. Gardner, J. J. Spivey, E. L. Kugler, A. Campos,
J. C. Hissam and A. D. Roy, J. Phys. Chem. C, 2010, 114,
7888–7894.
19 T. Montini, A. M. Condo, N. Hickey, F. C. Lovey, L. De Rogatis,
P. Fornasiero and M. Graziani, Appl. Catal., B, 2007, 73, 84–97.
20 H. Yin, Z. Ma, M. Chi and S. Dai, Catal. Today, 2011, 160,
87–95.
21 X. Zhang, H. Zhu, Z. Guo, Y. Wei and F. Wang, Int. J.
Hydrogen Energy, 2010, 35, 8841.
40 Y. L. Hou and S. Gao, J. Mater. Chem., 2003, 13, 1510–1512.
41 T. Noguchi and M. Sugiura, Biochemistry, 2003, 42,
6035–6042.
42 C. Aliaga, J. Y. Park, Y. Yamada, H. S. Lee, C. K. Tsung,
P. D. Yang and G. A. Somorjai, J. Phys. Chem. C, 2009, 113,
6150–6155.
22 W. Stöber, A. Fink and E. Bohn, J. Colloid Interface Sci.,
1968, 26, 62–69.
23 T. Liu, D. Li, Y. Zou, D. Yang, H. Li, Y. Wu and M. Jiang,
J. Colloid Interface Sci., 2010, 350, 58.
43 D. Karthikeyan, N. Lingappan, B. Sivasankar and
N. J. Jabarathinam, Ind. Eng. Chem. Res., 2008, 47, 6538–6546.
44 K. W. Wang, S. R. Chung and C. W. Liu, J. Phys. Chem. C,
2008, 112, 10242–10246.
24 C. Z. Wu, Z. Y. Lim, C. Zhou, W. G. Wang, S. H. Zhou,
H. F. Yin and Y. J. Zhu, Chem. Commun., 2013, 49, 321.
25 Y. B. Hu, K. Tao, C. Z. Wu, C. Zhou, H. F. Yin and S. H. Zhou,
J. Phys. Chem. C, 2013, 117, 8974–8982.
26 M. Koussathana, D. Vamvouka, H. Economou and X. Verykios,
Appl. Catal., 1991, 77, 283–301.
45 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou,
R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl.
Chem., 1985, 57, 603–619.
46 C. A. Marques, M. Selva and P. Tundo, J. Org. Chem., 1993,
58, 5256–5260.
47 L. Prati and M. Rossi, Appl. Catal., B, 1999, 23, 135–142.
48 Y. P. Zhang, S. J. Liao and Y. Xu, Tetrahedron Lett., 1994, 35,
4599–4602.
27 P. K. Cheekatamarla and A. M. Lane, Int. J. Hydrogen Energy,
2005, 30, 1277–1285.
28 N. Lingaiah, P. S. S. Prasad, P. K. Rao, L. E. Smart and
F. J. Berry, Appl. Catal., A, 2001, 213, 189–196.
29 Y. Li, Z. G. Li and R. X. Zhou, J. Mol. Catal. A: Chem., 2008,
279, 140–146.
30 A. Corma, P. Serna, P. Concepcion and J. J. Calvino, J. Am.
Chem. Soc., 2008, 130, 8748–8753.
49 C. V. Rode, M. J. Vaidya and R. V. Chaudhari, Org. Process
Res. Dev., 1999, 3, 465–470.
50 S. F. Wang, Y. H. Ma, Y. J. Wang, W. Xue and X. Q. Zhao,
J. Chem. Technol. Biotechnol., 2008, 83, 1466–1471.
51 S. Dutta, S. Sarkar, C. Ray, A. Roy, R. Sahoo and T. Pal,
ACS Appl. Mater. Interfaces, 2014, 6, 9134–9143.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2014