A hybrid nanocomposite precursor route to synthesize dispersion-enhanced Ni catalysts for the selective hydrogenation of o-chloronitrobenzene

Article abstract of DOI:10.1039/c2cy20597a

Highly-dispersed Ni nanoparticles over carbon nanotubes (CNTs) were successfully prepared from a hybrid nanocomposite of Ni-Al layered double hydroxide (NiAl-LDH) and poly acrylic acid (PAA) functionalized CNTs. The physicochemical properties of materials

Full text of DOI:10.1039/c2cy20597a

Catalysis
Science & Technology
View Article Online
View Journal | View Issue
PAPER
A hybrid nanocomposite precursor route to synthesize
dispersion-enhanced Ni catalysts for the selective
hydrogenation of o-chloronitrobenzene†
Cite this: Catal. Sci. Technol., 2013,
, 982
3
Jia Wang, Guoli Fan and Feng Li*
Highly-dispersed Ni nanoparticles over carbon nanotubes (CNTs) were successfully prepared from a
hybrid nanocomposite of Ni–Al layered double hydroxide (NiAl-LDH) and poly acrylic acid (PAA)
functionalized CNTs. The physicochemical properties of materials were investigated by powder X-ray
diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption
(
TPD), transmission electron microscopy (TEM), low-temperature nitrogen adsorption–desorption
experiments and X-ray photoelectron spectroscopy (XPS). The results revealed that L-cysteine as a
bridging linker could facilitate the immobilization of NiAl-LDH crystals onto the surface of
functionalized CNTs, thus giving rise to the dispersion-enhanced Ni nanoparticles with a small diameter
of about 6.0 nm and narrow size distribution in the range of 3–7 nm. Compared with that prepared by
the conventional impregnation method, the as-prepared supported Ni catalyst exhibited superior
catalytic performance in liquid phase selective hydrogenation of o-chloronitrobenzene to o-chloroaniline
along with the highest yield of 98.1% in 150 min, owing to its higher dispersion as well as the existence
of both the electron-rich support and electron-deficient metal species.
Received 26th August 2012,
Accepted 4th December 2012
DOI: 10.1039/c2cy20597a
www.rsc.org/catalysis
2
that TiO supported Ni catalysts and amorphous Ni–Fe–P alloys
Introduction
could display good catalytic performance in the hydrogenation
Nowadays, aromatic haloamines are extensively used as of o-chloronitrobenzene (o-CNB) to o-chloroaniline (o-CAN).12,13
chemical intermediates in the manufacture of pesticides, Especially, much attention has been paid to the enhancement
1
pharmaceuticals, pigments, herbicides and cosmetic products.
of dispersion of Ni catalysts in order to further improve their
Hydrogenation of halogenated nitro-aromatics, a main route catalytic performance. On the other hand, as a sustainable
for the synthesis of haloamine compounds, is often accompa- (clean and continuous) process for the production of commer-
nied by the undesired side reaction of hydrodehalogenation, cially important chemicals, gas phase hydrogenation of aro-
which can significantly reduce the yield of desired products. matic nitro compounds over supported Au and Ni catalysts has
Many efforts have been devoted in performing the liquid phase been performed by Keane and coworkers.1
4–16
hydrogenation of halogenated nitro compounds over a variety
In the hydrogenation of chloronitroarene, the effect of metal
catalysts on the activity and selectivity may be interpreted by
2
3
4,5
6,7
of supported nano-sized noble metal (e.g. Ag, Au, Pt, Pd,
8
and Ru ) catalysts, which show excellent selectivity to halogenated several geometric and/or electronic factors including metal particle
aniline and fast reduction rate to nitro groups. The high cost, size, incorporation of a second metal, metal-support interaction
17–30
however, limits their practical applications. At present, nickel- and metal dispersion.
based catalysts have been widely investigated in some catalytic larger particle size of metallic Pt in the Pt/Al O catalysts favored
For example, Coq et al. reported that the
2
3
hydrogenation reactions owing to their high activity, good the hydrogenation of nitro groups in the p-CNB molecule, due to
9
–11
selectivity and relatively low cost.
For instance, it is reported the large particles being more electron-rich and having multi-sites
1
7
for adsorption. Han et al. also found that electron deficient
species of the second elements (e.g. transition metals or rare
earths) could promote the specific activity of Pt/ZrO and Pt/TiO
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, P. O. BOX 98, Beijing, 100029, P.R. China.
2
5,26
2
2
catalysts by activating the nitrogen–oxygen bond.
Layered double hydroxides (LDHs), represented by the
E-mail: lifeng@mail.buct.edu.cn; Fax: +8610-64425385; Tel: +8610-64451226
†
Electronic supplementary information (ESI) available: Additional experimental
2
+
3+
x+
nꢀ
results. See DOI: 10.1039/c2cy20597a
general formula [M1ꢀx
M
x
(OH)
2
] [Ax/n
]
2
ꢁmH O, belong to
9
82 Catal. Sci. Technol., 2013, 3, 982--991
This journal is c The Royal Society of Chemistry 2013

View Article Online
Paper
Catalysis Science & Technology
3
1–33
a family of two-dimensional layered materials,
where fundamental insights into the catalytic mechanism of this type
2
+
3+
different M and M metal cations uniformly distribute and of highly dispersed Ni nanoparticles.
orderly prearrange in the brucite-like sheets, and various
charge-compensating anions (A ) are present in the interlayer
space. According to the fact that LDHs can accommodate a
large number of tunable M and M ions on the layers or in
the interlayer space in the form of metal complexes, well-
nꢀ
Results and discussion
Synthesis and microstructure of highly dispersed Ni
nanoparticles over CNTs
3
4
2
+
3+
dispersed supported metal catalysts may be formed by reducing Fig. 1 shows the XRD patterns of LDH/P-CNTs and LDH-L/P-CNTs
3
5–38
calcined LDHs.
This kind of structural transformation nanocomposites. All characteristic reflections can be ascribed
endows LDH materials with extraordinary capability as catalyst to graphite (JCPDS No. 41-1487) and hydrotalcite-like LDH
3
9–43
precursors in various metal-catalyzed reactions.
On the (JCPDS No. 15-0087) phases. The two sharp and intense reflec-
other hand, carbon nanotubes (CNTs) are very attractive as tions appearing as symmetric lines at 2y angles below 251 are
catalyst supports in heterogeneous catalysis due to their out- attributed to (003) and (006) planes of the NiAl-LDH phase. The
standing properties, such as high aspect ratio, large surface basal spacing value of d003 corresponds to the lattice parameter
area, excellent chemical and thermal stability, extremely high c (= 3 ꢂ d003), while the (110) reflection of the NiAl-LDH
mechanical strength, and good electronic conductivity. To date, structure allows to calculate the lattice parameter a (= 2 ꢂ d110).
4
4,45
46
47
a variety of metals and metal oxides, such as Pt,
Pd, and RuO ,
2
Au, Ru,
The a and c of the NiAl-LDH phase in two samples are
have been deposited onto CNTs to form approximately 3.03 and 23.15 Å, respectively. According to the
4
8
49
versatile supported catalysts for achieving excellent catalytic Scherrer equation, compared with that in LDH/P-CNTs, the
performance. The adhesion of guest materials on CNTs matrix, average crystallite size calculated by the Scherrer equation in
however, is often not strong enough to survive the mechanical both the a and c direction of NiAl-LDH for LDH-L/P-CNTs is
shaking involved in reaction, owing to the conventional pre- decreased from 7.1 to 5.6 nm and 7.5 to 4.1 nm, respectively.
paration method, i.e., simple impregnation of CNTs into the This may be ascribed to the segregation and inhibition effect
starting salt solution and subsequent reduction. The applied of L-cysteine as a bridging linker on the growth of NiAl-LDH
5
1
strategy for efficient immobilization of active species onto crystals.
CNTs is commonly to utilize surface modification of CNTs with
The reducibility of LDH/P-CNTs and LDH-L/P-CNTs samples
functional groups or organic binders for enhancing the inter- was investigated by TPR. However, if a non-selective TCD signal
5
0
action between host CNTs and desired components.
is used, there are certainly the signals given by other released
In this work, we established a novel route to synthesize gases except for H . To understand the effect of the released
2
dispersion-enhanced Ni nanoparticles over CNTs, which were gases produced by the decomposition of samples on TPR
derived from NiAl-LDH/poly acrylic acid (PAA) functionalized results, TPDE experiments of two nanocomposites were firstly
CNTs nanocomposite precursor. A schematic illustration for carried out. There is a tiny decomposition peak centered at
the synthesis of highly dispersed Ni nanoparticles is presented 361 1C (Fig. 2a), which corresponds to the decarbonation of
in Scheme 1. The catalytic performance of the as-synthesized the interlayer carbonate anions for the NiAl-LDH phase in
Ni catalyst was studied in liquid phase hydrogenation of o-CNB LDH/P-CNTs, and another small peak at 421 1C is ascribed to
to o-CAN. A correlation between the microstructure of catalysts the decomposition of PAA. For LDH-L/P-CNTs, except for a
and the catalytic performance was investigated, which gave
Fig. 1 XRD patterns of nanocomposite precursors: (a) LDH/P-CNTs and (b) LDH-
Scheme 1 Schematic illustration for the synthesis of highly dispersed Ni nano-
L/P-CNTs. Inset shows an enlargement of fitted (110) and (113) reflections of the
NiAl-LDH phase.
particles over CNTs.
This journal is c The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 982--991 983

View Article Online
Catalysis Science & Technology
Paper
Fig. 3 XRD patterns of (a) Ni/P-CNTs and (b) Ni-L/P-CNTs.
oxide (Bunsenite, JCPDS No. 47-1049) phases are detected.
Meanwhile, the broad asymmetric peak at about 441 can be
deconvoluted into two peaks with their maxima at 43.3 and
Fig. 2 TPDE and TPR profiles of LDH/P-CNTs (a and c) and LDH-L/P-CNTs (b and d).
44.51, corresponding to the (200) reflection of nickel oxide and
the (111) reflection of the metallic nickel, respectively. Com-
pared with that in Ni/P-CNTs, a much larger amount of metallic
nickel in Ni-L/P-CNTs is formed except for a little NiO phase,
probably owing to the uniform dispersion of smaller NiAl-LDH
crystals over P-CNTs. No characteristic reflections corresponding
to the Al O phase are observed in the XRD patterns of samples,
small decomposition peak of PAA at about 430 1C, there is a
broad and small peak at about 332 1C (Fig. 2b), which is
assigned to both the decarbonation of the NiAl-LDH phase
and the decomposition of L-cysteine. In addition, CNTs and
P-CNTs samples present negligible hydrogen consumption at
high temperature above 600 1C in the TPR profiles. Therefore,
the fact that the peaks due to the decomposition of NiAl-LDH,
PAA and L-cysteine in the nanocomposites are small allows
performing a classic TPR characterization in the present system.
In the TPR profile of LDH/P-CNTs (Fig. 2c), there is a wide and
large reduction peak centered at 470 1C accompanied with a
small shoulder at around 376 1C. The main peak is attributed to
2
3
55
suggesting the existence of an amorphous alumina phase.
Correspondingly, the kind of amorphous Al O phase attaching
2
3
to Ni nanoparticles formed over support can prevent the
aggregation of Ni nanoparticles as a dispersant agent, thus
enhancing the dispersion of Ni nanoparticles. In addition,
X-ray line-broadening analyses from the (111) reflection of the
metallic nickel phase yield a decreasing average particle size
from Ni/P-CNTs to Ni-L/P-CNTs (Table 1).
TEM images of Ni-L/P-CNTs reveal that uniform nano-
particles are highly dispersed on the external surface of nano-
tubes, and the aggregation of the nanoparticles is less visible
2
+
the reduction of Ni interacting with near groups through the
3
+
Ni–O bonds polarized by Al ions in the mixed oxides formed by
the decomposition of NiAl-LDH in the composite, while the
2
+
small shoulder should be ascribed to the reduction of Ni in
5
2
the form of bulk crystallite NiO species. In addition, another
(Fig. 4a). A high-magnified TEM image (Fig. 4b) further reveals
peak appearing at about 321 1C may be attributable to the
the relatively thin and faceted nanoparticles (indicated by the
dark arrows), which is characteristic of the interaction between
3
+
reduction of a small amount of Ni species on the external
5
3
layers of solids. For LDH-L/P-CNTs, however, only a single
sharp reduction peak is observed at the lower temperature of
56
nanoparticles and support. Meanwhile, nanoparticles present a
narrow distribution with the diameter (D) of about 6.0 nm (Fig. 4c).
2
60 1C (Fig. 2d), which could be due to the smaller crystallite size
2
+
of LDH in the LDH-L/P-CNTs leading to easier reduction of Ni
species. The reason is further discussed in detail in the following
sections. On the other hand, the molar ratio of the consumed H
Table 1 Analytical and structural data obtained for different samples
2
c
V
engaged in the TPR reduction process to the total amount of
nickel in the LDH-L/P-CNTs and LDH/P-CNTs is about 0.98 and
a
b
d
e
f
D
111
S (ꢃ5 (ꢃ0.01
d
D
Dis
2
ꢀ1
3
ꢀ1
Sample
(ꢃ0.5 nm) m g ) cm g ) (ꢃ0.5 nm) (ꢃ0.9 nm) (ꢃ0.4%)
1.04, respectively, quite close to the theoretical ratio of 1. Mean-
CNTs
P-CNTs
—
—
95
89
109
206
0.42
0.37
0.42
0.31
16.2
16.9
11.6
5.6
—
—
—
6.0
—
—
11.8
20.5
while, there is not any detectable H2 desorption peak above
7
00 1C after the TPR process followed by a H -TPD process. Ni/P-CNTs 17.0
2
Ni-L/
P-CNTs
a
9.2
The results suggest that the possible involvement of spillover
5
4
hydrogen in the TPR measurements can be disregarded.
Crystallite size based on XRD line broadening of the (111) plane for
Fig. 3 depicts the XRD patterns of Ni/P-CNTs and Ni-L/P-CNTs
samples, where metallic nickel (JCPDS No. 04-0850) and nickel
b
c
d
Ni metal. BET surface area. Total pore volume. Average pore diameter.
e
f
Particle size calculated by TEM analysis. Degree of Ni metal dispersion.
9
84 Catal. Sci. Technol., 2013, 3, 982--991
This journal is c The Royal Society of Chemistry 2013

View Article Online
Paper
Catalysis Science & Technology
Fig. 4 TEM images of Ni-L/P-CNTs (a,b), histogram of the size distribution of
nanoparticles in Ni-L/P-CNTs (c), HRTEM images of Ni-L/P-CNTs (d), a single
crystalline Ni nanoparticle on CNTs (e) and TEM image of Ni/P-CNTs (f). Inset in
(
f) shows a magnified image of Ni/P-CNTs.
In contrary, the Ni/P-CNTs sample shows very poor dispersion
of relatively larger nanoparticles with severe aggregation
Fig. 5 XPS spectra of C 1s (A), Ni 2p (B), Al 2p (C), and O 1s (D) regions in NiAl-
LDH (a) and LDH-L/P-CNTs (b).
(Fig. 4f). HRTEM images of Ni-L/P-CNTs clearly indicate the
lattice fringes with the interplanar spacings of about 2.03 Å for
a single nanoparticle (Fig. 4d and e), corresponding to the (111) existence of a large amount of functional groups (–COO ) in
ꢀ
plane of fcc metallic nickel phase. H
2
-TPD results show that the PAA chains on the surface of support. As for the NiAl-LDH
dispersion of Ni/P-CNTs and Ni-L/P-CNTs are about 11.8 and sample, the peak of C 1s at 288.6 eV is attributed to the CQO in
2
ꢀ
2
0.5%, respectively, in good agreement with the TPR and TEM CO3 . Moreover, the BE value of Ni 2p is 856.0 eV, indicative
3/2
2
+
62
results.
of the presence of Ni species bonded to hydroxyl group. The
In the case of Ni-L/P-CNTs, this high affinity and dispersion fine spectrum of Al 2p shows a peak with BE of 74.1 eV, related
3
+
63
nature of Ni nanoparticles should originate from the inter- to the Al species in the form of Al–OH. From the O1s
action between the NiAl-LDH phase and support through the spectrum for NiAl-LDH, it is seen that only one fitted peak at
L-cysteine bridging linker in the synthesis of the LDH-L/P-CNTs 531.6 eV appears, originating from lattice oxygen species.
precursor. L-cysteine, which contains a large number of amino Compared with those for NiAl-LDH, however, the BE values of
ꢀ
+
and carboxyl groups in the ionic form of –(COO )NH
3
in acidic Ni 2p, Al 2p, and O 1s for LDH-L/P-CNTs all shift to the much
5
7
salt solution, can be immobilized onto the surface of P-CNTs higher values of 857.2, 75.4, and 532.7 eV, respectively, which
through the electrostatic interaction between surface groups of should be ascribed to the charge transfer from the NiAl–LDH
L-cysteine and functional groups (–COO ) in PAA chains, as crystallites to the surface of P-CNTs due to the well-dispersed
ꢀ
58
ꢀ
illustrated in Scheme 1. Herein, –COO groups of immobilized nature of LDH nanoparticles involving the electronic inter-
L-cysteine can coordinate selectively and/or interact electro- action between them in composites.
2
+
3+
statically with Ni and Al cations in the solution, which
Fig. 6 shows the nitrogen adsorption–desorption isotherms
facilitates the adhesion and dispersion of NiAl-LDH crystals onto and pore size distribution curves for CNTs, P-CNTs, Ni/P-CNTs
P-CNTs and thus leading to the formation of highly dispersed and Ni-L/P-CNTs. All the isotherms can be classified as type IV
Ni nanoparticles. Such an influence of surface carboxylate groups category indicating the presence of capillary condensation in
of the support on the dispersion of Ni nanoparticles is similar to the mesoporous structure. CNTs and P-CNTs exhibit two pore
those earlier reports about rhodium and gold nanoparticles size distributions ranging from 2 to 5 and 10–100 nm, which
5
9,60
stabilized by carboxylate modified polyvinylpyrrolidone.
may be the result of the exposure of the inside pores of
The surface chemical states for NiAl-LDH and LDH-L/P-CNTs nanotubes and the condensation in the void fraction of tangled
are analyzed by XPS, as shown in Fig. 5. The C1s spectrum nanotubes, respectively. Compared with CNTs, the P-CNTs
of LDH-L/P-CNTs can be deconvoluted into five separate support shows a decrease in pore volume (Table 1) due to the
6
1
components. The main peak at about 284.6 eV is assigned to coating of nanotubes with PAA. Ni/P-CNTs shows a H1 hysteresis
2
2
the sp -hybridized graphite-like carbon atoms and sp carbon loop associated with the aggregates of plate-like particles giving
6
4
atoms bound to hydrogen. The peak at around 285.3 eV rise to slit-shaped mesopores, whereas Ni-L/P-CNTs presents
originates from defects on the graphitic structure. The other a H4 hysteresis loop indicating the presence of narrow meso-
two peaks centered at about 286.5 and 288.9 eV are attributed to pores having uniform size and/or shape. Compared with that of
C–O and CQO, respectively. The small peak for carbon p–p* P-CNTs, the pore size distribution of Ni-L/P-CNTs covers a
transitions also presents at about 290.8 eV. It suggests the narrower range (around 2–10 nm), with a maximum at about
This journal is c The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 982--991 985

View Article Online
Catalysis Science & Technology
Paper
Fig. 6 Nitrogen adsorption–desorption isotherms of CNTs and P-CNTs (a), Ni/P-CNTs
Fig. 7 XPS spectra of Ni/P-CNTs (a) and Ni-L/P-CNTs (b) in the Ni 2p region at
and Ni-L/P-CNTs (b). Insets show pore size distribution curves of samples.
different ion sputtering times.
3
.7 nm (inset in Fig. 6b), suggesting that the Ni nanoparticles
0
which indicates that there are more Ni species inside of
have been formed densely on the surface of CNTs. However,
Ni/P-CNTs sample presents a larger pore diameter with a
maximum at 11.9 nm, which implies the presence of a new
mesoporous structure due to severe aggregation of Ni-containing
species, as convinced by the TEM observation in Fig. 4f. More
0
samples due to the easy oxidation of Ni species outside of
2+
0
samples upon exposure in air. The Ni /Ni molar ratio at a
sputtering time of 480 s decreases from 4.29 for Ni/P-CNTs s to
.86 for Ni-L/P-CNTs, which is consistent with the above XRD
and TPR results. On the other side, compared with Ni/P-CNTs
and Ni-L/P-CNTs, IM-Ni/P-CNTs has a higher reduction degree
Table 2). In the case of LDH-derived samples, Ni species
related to the NiO phase can interact with near groups through
the Ni–O bonds polarized by Al ions in the metal oxides
formed by the decomposition of surface LDH phase, thus
hindering the reduction of metal species.
0
2
ꢀ1
significantly, the BET surface area of Ni-L/P-CNTs (206 m g ) is
about two times higher than that of Ni/P-CNTs (109 m
2
ꢀ1
g
)
2+
(
(Table 1), confirming the enhanced dispersion of smaller Ni
nanoparticles on the support.
The XPS spectra of Ni/P-CNTs and Ni-L/P-CNTs in the Ni 2p
region at different ion sputtering times are shown in Fig. 7. Two
3+
0
2+
peaks at 853.7 eV and 857.2 eV are assigned to Ni and Ni
species, respectively. It is worth noting that the BE values of
Ni species are much higher than those reported for Ni species
0
Catalytic property of highly dispersed Ni nanoparticles
2
+
(
about 852.7 eV) and Ni species (about 854.6 eV) in the The catalytic hydrogenation of o-CNB in the liquid phase is an
6
5
literature, which are the same as those for IM-Ni/P-CNTs example of a three-phase catalytic reaction, and therefore,
Fig. S1, ESI†). The result is probably ascribed to be the charge several aspects have to be considered in order to achieve a
(
transfer from the surface of metallic nickel and nickel oxide to proper and efficient catalyst testing: gas–liquid (G–L), liquid–
support, indicative of the existence of an electronic interaction solid (L–S) and internal diffusion. In the present system,
between metal and support. The similar phenomenon also was Ni/P-CNTs, Ni-L/P-CNTs and IM-Ni/P-CNTs catalysts do not
6
6
2
found in a TiO /CNTs composite system. Meanwhile, during present both internal and external mass transport limitations
ion sputtering, it can be found that the relative percentage of in the hydrogenation of o-CNB and thus the intrinsic activity of
0
Ni for samples increases from Ni/P-CNTs to Ni-L/P-CNTs. From the catalysts is actually assessed under our reaction conditions
0
Fig. 7, Ni content increases with the increase of ion sputtering time, (see ESI†). Another test was carried out in order to determine
9
86 Catal. Sci. Technol., 2013, 3, 982--991
This journal is c The Royal Society of Chemistry 2013

View Article Online
Paper
Catalysis Science & Technology
Table 2 Catalytic performances for hydrogenation of o-CNB over different catalysts
a
Content (wt%)
Degree of
reduction
(ꢃ0.5%)
Ni metal
dispersion
(ꢃ0.4%)
Selectivity (%)
Initial reaction
rate (mmol s
Conversion of
c
b
ꢀ1 Niꢀ1
d
Catalyst
Ni (ꢃ0.5%)
Al (ꢃ0.5%)
g
)
o-CNB (%)
o-CAN
AN
Others
Ni/P-CNTs
Ni-L/P-CNTs
IM-Ni/P-CNTs
20.6
21.1
20.7
3.0
3.2
—
46
95
100
11.8
20.5
6.1
0.39
35.9
24.5
2.4
99.3
48.2
98.5
98.8
93.8
0
1.1
5.3
1.5
0.1
0.9
a
b
c
d
As determined by element analysis. Reaction temperature: 140 1C. Reaction time: 150 min. By-products: CNSB and CPHA.
the possibility of hydrogen transfer from solvent to the reac- However, the yield of o-CNB is only 45.2% in 150 min over
tants. This was disregarded since the conversion of o-CNB was the IM-Ni/P-CNTs catalyst prepared by the impregnation
nil for the catalytic test in the absence of H . It is well method, where the severe aggregation of large Ni particles
2
documented that the catalytic hydrogenation of o-CNB usually can be observed (Fig. S3, ESI†). In addition, IM-Ni/P-CNTs with
goes through a complex multi-step reaction pathway (Fig. S2, much lower dispersion shows a higher conversion level and
ESI†) and the desired product is o-CAN. The main side reaction, initial reaction rate than Ni/P-CNTs, which may be due to the
hydrodechlorination of o-CNB and o-CAN, can produce aniline lower reduction degree of Ni in Ni/P-CNTs. On the other hand,
3
(AN) through the activation of the C–Cl bond by the aromatic NH -TPD profiles of catalysts (Fig. S4, ESI†) reveal the genera-
ring. Meanwhile, the accumulation of intermediates including tion of broad high temperature desorption signals in the range
o-chloronitrosobenzene (CNSB) and N-chloro-phenylhydroxyl- of 300–650 1C, indicative of the presence of strong Br o¨ nsted
amine (CPHA) can not be neglected.
The hydrogenation of o-CNB was carried out at 140 1C under (–O–H) of CNTs. The overall acid amounts for catalysts based
a hydrogen partial pressure of 2.0 MPa. The conversion of on the NH -TPD results appear to obey the following trend:
acid sites related to the presence of surface oxygenated groups
6
7
3
o-CNB and the selectivity to o-CAN over different supported Ni-L/P-CNTs 4 IM-Ni/P-CNTs 4 Ni/P-CNTs. As a result, in a way,
catalysts are plotted in Fig. 8, while the results are summarized surface acidity also has a positive effect on the hydrogenation
in Table 2. Among the catalysts, Ni-L/P-CNTs exhibits the reaction, which is probably ascribed to the intermolecular
highest initial reaction rate. And 99.3% conversion of o-CNB interaction between the reactant and the surface acid group
6
8
and 98.8% selectivity to o-CAN in 150 min are achieved over acting as a proton donor.
such a catalyst. It is worth noting that the initial reaction rate
As listed in Table 2, compared with Ni/P-CNTs, the Ni-L/P-CNTs
ꢀ
1
ꢀ1
0
reaches 35.9 mmol s
g
Ni
over the Ni-L/P-CNTs catalyst, catalyst presents higher dispersion of Ni species due to the
which is almost 90 times larger than that obtained over the larger surface area and the smaller particle size, which pro-
ꢀ1
ꢀ1
Ni/P-CNTs catalyst (0.39 mmol s
gNi ) at the same reaction motes the contact of reactants with active sites on catalyst.
conditions. Compared to other Ni-based catalysts reported in Usually, as for hydrogenation catalyzed by metals, the adsorp-
the literature, e.g. filamentous carbon stabilized nickel catalysts tion nature is the key factor for the selectivity. According to the
1
1,13
and amorphous Ni–Fe–P alloys,
Ni-L/P-CNTs catalyst shows above XPS results, the electron transfer from metallic Ni to
higher hydrogenation reaction rate. The above results clearly graphite edges of nanotubes demonstrates the presence of
demonstrate the high activity of the Ni-L/P-CNTs catalyst. a strong metal-support interaction (SMSI) in LDHs-derived
catalysts. Such SMSI can increase the electronic density for
p-electrons of the support and thus induces an enhanced
oriented chemisorption of o-CNB with the electron withdrawing
6
9
nitro-group by repulsion of the negative part of o-CNB, which is
located on the chlorine site. Correspondingly, the hydrogenolysis
of the C–Cl bond in o-CNB and o-CAN is inhibited, leading
to high selectivity to o-CAN (498%) over Ni/P-CNTs and
Ni-L/P-CNTs catalysts. As for the IM-Ni/P-CNTs catalyst, the
relatively low selectivity towards the desired product measured
should originate from the presence of bulky nickel particles on
the support leading to the low metal dispersion and thus weak
interaction between metal–support.
As shown in Fig. 9a, elevating reaction temperature from
100 to 140 1C results in continuous increase in the conversion
of o-CNB and the selectivity to o-CAN under the hydrogen
partial pressure of 2.0 MPa, because the rate of primary reac-
tion can be enhanced with the reaction temperature. Further
increase in the reaction temperature from 140 to 200 1C slightly
improves the conversion of o-CNB. In contrast, the selectivity to
Fig. 8 Effect of reaction time on the conversion of o-CNB and the selectivity to
o-CAN over different catalysts at 140 1C.
This journal is c The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 982--991 987

View Article Online
Catalysis Science & Technology
Paper
Interestingly, Ni-L/P-CNTs can keep its initial crystallo-
graphic structure even after long term exposure in air for 30 d
(Fig. S5, ESI†), indicating its excellent structural and chemical
stability. Furthermore, Ni-L/P-CNTs can maintain its excellent
catalytic activity (reaching the o-CAN yield of 95%) at 140 1C
under the hydrogen partial pressure of 2.0 MPa after recycling
for 5 times. The slight decrease in the yield of o-CAN may be
due to the chlorine retention during the reaction, which
provokes the partial nickel shielding. Elemental analysis by
ICP-AES further reveals that the Ni leaching loss is only about
0.3 wt% for the Ni-L/P-CNTs sample after recycling for 5 times,
suggestive of the good stability of the catalyst. The above result
suggests that the catalyst synthesized by the present method is
both chemically and structurally stable.
Conclusions
A dispersion-enhanced nickel nanoparticles over CNTs, Ni-L/P-CNTs,
was prepared from a hybrid nanocomposite of NiAl-LDH and
PAA-functionalized CNTs. In Ni-L/P-CNTs, Ni nanoparticles with
a narrow size distribution centered at around 6.0 nm were highly
dispersed on the external surface of the support. Ni-L/P-CNTs
exhibited superior catalytic performance in liquid phase hydro-
genation, compared with that prepared by the conventional
impregnation method. The unusual behavior of Ni-L/P-CNTs is
0
attributed to high dispersion of Ni species, as well as the
existence of an electronic interaction between metal and sup-
port modifying the mode of the metal-reactant interaction and
thus enhancing the selectivity towards the desired products.
Moreover, increasing reaction temperature and the Ni content
could improve the conversion of o-CNB by accelerating the rate
of hydrogenation, but higher reaction temperature and Ni
content led to the decrease in the selectivity of o-CAN due to
the enhanced side reactions. Owing to the intrinsic properties
such as high surface area, chemical/thermal stability and low
cost, it is expected that similar to other supported catalysts
Fig. 9 Effects of reaction temperature (a) and Ni content (b) on the conversion
of o-CNB and the selectivity to o-CAN over Ni-L/P-CNTs with a reaction time of reported previously, such a type of highly dispersed and stable
1
50 min.
Ni catalyst may work as an alternative to noble metals, which is
probably appropriate in continuous gas phase hydrogenation
for the practical production of haloamines.
o-CAN displays a dramatic decrease, which is ascribed to the
enhanced rate of side reactions with elevated temperatures.
Increasing the amount of catalyst in batch reaction processing
will increase the conversion due to more active sites provided
Experimental
Materials
by catalysts, but this positive effect is not always to be utilized if Multi-walled CNTs (purity 495%) with outer diameters of
the selectively of the product is considered. As shown in Fig. 9b, 20–40 nm were purchased from Shenzhen Nanotech Port Co.,
the conversion of o-CNB over Ni-L/P-CNTs is improved with the Ltd. Acrylic acid (AA, 99.5%), 2,2-azobisisobutyronitrile (AIBN,
Ni content from 6.8 to 28.9 wt% in 150 min. After a slow 99.5%), acetone (99.5%), L-cysteine (98.5%), o-CNB (99%) and
increase of selectivity in the cases of lower Ni contents, the other chemicals (analytical grade) were used without further
selectivity to o-CAN decreases from 98.0 to 91.7% with the Ni purification.
content increasing from 21.1 to 28.9 wt%. This is because when
the amount of active sites increases, the higher conversion
leads to a relatively lower concentration of o-CNB in the
Preparation of PAA functionalized CNTs
reaction solution. However, the excess of Ni active sites are As-received CNTs (1.0 g) were added into acetone (200 mL) in an
available for the adsorption of o-CAN, which may further ultrasonic bath for 10 min. Subsequently, AA (10 mL) was
convert to AN leading to the decrease in the selectivity to o-CAN. added into the solution under nitrogen atmosphere for 30 min.
9
88 Catal. Sci. Technol., 2013, 3, 982--991
This journal is c The Royal Society of Chemistry 2013

View Article Online
Paper
Catalysis Science & Technology
Then AIBN (0.5 g) was added into the above solution, which was (H
2
-TPR) and temperature programmed decomposition (TPDE)
refluxed and vigorously stirred at 55 1C for 8 h. The suspension using a Micromeritics ChemiSorb 2720. For H
2
-TPR, 0.1 g of
was cooled, filtered and washed thoroughly with deionized precursor, which was placed in a quartz U-tube reactor, was
ꢀ1
water, and then dried under vacuum at 50 1C for 12 h. The degassed at 200 1C for 2 h under argon flow (40 mL min ). Then
ꢀ
1
obtained fine powder was denoted as P-CNTs.
TPR was conducted in a stream of 10% v/v H /Ar (40 mL min )
2
ꢀ1
with a heating rate of 5 1C min up to 800 1C. For TPDE, 0.1 g
of nanocomposite precursor was used only under argon flow
Nanocomposite of NiAl-LDH and P-CNTs was assembled by a (40 mL min ) with a heating rate of 5 1C min up to 800 1C.
co-precipitation method. Firstly, Ni(NO ) ꢁ6H O (15 mmol) and The effluent gas was analyzed by a thermal conductivity
Preparation of highly-dispersed Ni nanoparticles over CNTs
ꢀ1
ꢀ1
3
2
2
Al(NO ) ꢁ9H O (5 mmol) were dissolved in deionized water detector (TCD).
3
3
2
(50 mL). Then P-CNTs (0.5 g) and L-cysteine (0.9087 g) were
Ammonia temperature-programmed desorption (NH -TPD)
3
dispersed into the solution in an ultrasonic bath for 30 min experiments were carried out using a Micromeritics ChemiSorb
under nitrogen atmosphere. Subsequently, the solution was 2720. The sample (0.2 g) was degassed by heating under He
titrated with an alkali solution of NaOH (0.64 M) and Na
0.2 M) under vigorous stirring at room temperature until ammonia gas mixture (5% NH
pH = 10.0. The resulting suspension was aged at 60 1C with Chemisorbed NH was desorbed by heating from the saturation
2
CO
3
environment at 500 1C for 1 h and then treated with flowing
(
3
in He) for 1.5 h at 120 1C.
3
ꢀ1
further stirring for 6 h. The solid was then recovered and temperature up to 700 1C at a rate of 10 1C min
washed in deionized water and finally dried under vacuum at
The reduction degree of reduced samples (Ni/P-CNTs and
0 1C for 12 h. The obtained sample was hereafter denoted as Ni-L/P-CNTs) was determined by H -TPR. First, nanocomposite
.
6
2
LDH-L/P-CNTs, where L represents L-cysteine. For comparison, precursor (0.1 g) was reduced in the U-shaped quartz reactor at
ꢀ
1
pristine NiAl-LDH was prepared under identical reaction con- 500 1C (heating rate 5 1C min ) for 2 h in flowing 10% v/v
ꢀ
1
ditions without the addition of L-cysteine and P-CNTs, and
H
2
/Ar (40 mL min ), which was the same condition as that for
/Ar gas was
LDH/P-CNTs sample was also prepared under identical reaction preparing the reduced samples. Then, the H
conditions without the addition of L-cysteine. LDH-L/P-CNTs switched to Ar gas (40 mL min ). After 30 min, the flowing
2
ꢀ1
and LDH/P-CNTs nanocomposites were placed in a tube- gas was changed to 10% v/v H /Ar again and the temperature
2
furnace reactor. The furnace was ramped from 30 to 500 1C at linearly increased from 500 up to 1000 1C at a heating rate of
ꢀ
1
ꢀ1
ꢀ1
a rate of 5 1C min under 10% v/v H
and held at 500 1C for 2 h. The obtained black products were decomposition of L-cysteine and the decarbonation of the
denoted as Ni-L/P-CNTs and Ni/P-CNTs, respectively. nanocomposite precursors proceeded completely during the
2
/Ar flow (40 mL min ), 10 1C min . On the basis of the fact that the oxidation or
For comparison, P-CNTs supported Ni sample with the Ni 2 h treatment at 500 1C, as confirmed by the TPDE profiles of
content of 21.0 wt% was also prepared by incipient wetness nanocomposite precursors, the reduction degree of samples
impregnation using nickel nitrate aqueous solution. The slurry was calculated by the percentage of the difference value to the
was dried at 80 1C overnight and calcined at 350 1C for 2 h in theoretical amount of hydrogen consumed, and the difference
air. Finally, the solid was reduced under 10% v/v H /Ar at 500 1C value was the theoretical amount of hydrogen consumed minus
2
ꢀ
1
for 2 h with a ramping rate of 5 1C min . The obtained sample the amount of hydrogen consumed corresponding to the
was referred to as IM-Ni/P-CNTs.
reduction of unreduced NiO phase which was measured by
the area of the TPR peaks from 500 to 1000 1C. Therefore, the
reduction degree (RD, %) of sample is estimated as the eqn (2):
Characterizations
Powder X-ray diffraction (XRD) patterns were determined on
a Shimadzu XRD-6000 diffractometer using Cu Ka radiation
ꢀ
ꢁ
ꢀ1
H2 uptake=mol gcat
ꢂ MNi=g molꢀ1 ꢂ 100;
RD ð%Þ ¼ 1 ꢀ
(l = 0.15418 nm).
%w=w Ni
Elemental analysis was performed using a Shimadzu ICPS-
ð2Þ
7500 inductively coupled plasma atomic emission spectrometer
ꢀ
1
(ICP-AES). Prior to analysis, the samples were dissolved in where MNi is the formula weight of Ni (M = 58.71 g mol ),
Ni
ꢀ1
chloronitrous acid, filtered, and diluted with deionized water.
H uptake (mol g
) is moles of H consumed from 500 to
2
2
cat
Transmission electron microscopy (TEM) and high-resolution 1000 1C per unit weight of reduced sample and %w/w Ni is the
TEM (HRTEM) were carried out on a JEOL 2100. The surface weight percentage of Ni in sample as determined by ICP-AES.
area-weighted diameter of particles (D) of samples was calculated
from more than 600 individual metal particles by the eqn (1):
The active metal surface of the reduced samples was
measured by H temperature programmed desorption (H -TPD)
2
2
P
nidi3
using Micromeritics ChemiSorb 2720. The nanocomposite pre-
cursor (0.1 g) firstly was reduced under the same condition as
that for preparing the reduced catalyst. The reduced sample
was cooled at 100 1C and degassed for 2 h under argon flow
i
D ¼ P
2;
ð1Þ
i
nidi
where n is the number of particles of diameter d .
The reduction behaviour and thermal decomposition of (40 mL min ), and H -TPD was carried out with a heating rate
2
LDH-L/P-CNTs and LDH/P-CNTs nanocomposite precursors of 10 1C min . Taking into account the reduction degree of
i
i
ꢀ
1
ꢀ
1
was studied by hydrogen temperature-programmed reduction sample and the presence of the desorption peak related to
This journal is c The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 982--991 989

View Article Online
Catalysis Science & Technology
Paper
5
4
spillover H
2
in the H
2
-TPD profiles above 700 1C, the Ni metal
3 Y. Chen, J. Qiu, X. Wang and J. Xiu, J. Catal., 2006, 242, 227.
4 J. L. Zhang, Y. Wang, H. Ji, Y. G. Wei, N. Z. Wu, B. J. Zuo and
Q. L. Wang, J. Catal., 2005, 229, 114.
5 X. Yuan, N. Yan, C. Xiao, C. Li, Z. Fei, Z. Cai, Y. Kou and
P. J. Dyson, Green Chem., 2010, 12, 228.
surface dispersion (Dis, %) was calculated as the eqn (3):
ꢀ1
H2 uptake=mol gcat ꢂ SF
Dis ð%Þ ¼
ꢂ MNi=g molꢀ1 ꢂ 100;
ð3Þ
%
w=w Ni ꢂ RD
6
7
8
9
G. Zhang, L. Wang, K. Shen, D. Zhao and H. S. Freeman,
Chem. Eng. J., 2008, 141, 368.
C. Liang, J. Han, K. Shen, L. Wang, D. Zhao and
H. S. Freeman, Chem. Eng. J., 2010, 165, 709.
J. Ning, J. Xu, J. Liu, H. Miao, H. Ma, C. Chen, X. Li, L. Zhou
and W. Yu, Catal. Commun., 2007, 8, 1763.
F. Alonso, P. Riente and M. Yus, Acc. Chem. Res., 2011,
catꢀ1
where H
2
uptake (mmol g
) below 700 1C is moles of H
2
desorbed per unit weight of reduced sample and SF is the
stoichiometric factor (the Ni/H = 1/1) which is taken as 2.
N adsorption–desorption of the samples was obtained on a
2
Micromeritics ASAP 2020 sorptometer apparatus. The specific
surface area was evaluated by the Brunauer–Emmett–Teller
BET) method. The pore size distribution was determined by
the Barrett–Joyner–Halenda (BJH) method applied to desorption
isotherm.
X-Ray photoelectron spectroscopy (XPS) of samples was
recorded on a VG ESCALAB 2201 XL spectrometer with a
monochromatic Mg Ka X-ray radiation (1253.6 eV photons).
The depth profiling experiment was conducted at argon partial
44, 379.
(
1
1
1
0 X. Meng, H. Cheng, S. Fujita, Y. Hao, Y. Shang, Y. Yu, S. Cai,
F. Zhao and M. Arai, J. Catal., 2010, 269, 131.
´
˜
1 N. Mahata, A. F. Cunha, J. J. M. Orfao and J. L. Figueiredo,
Catal. Commun., 2009, 10, 1203.
2 J. Chen, N. Yao, R. Wang and J. Zhang, Chem. Eng. J., 2009,
148, 164.
ꢀ
6
13 X. Yan, J. Sun, Y. Wang and J. Yang, J. Mol. Catal. A: Chem.,
2006, 252, 17.
pressure of 10 Torr, while applying 4 kV voltage at an ion
beam current of 10 mA cm , resulting in a B0.5 nm min
etching rate. Binding energies were calibrated based on the
graphite C 1s peak at 284.6 eV.
2
ꢀ1
1
1
1
1
4 F. C ´a rdenas-Lizana, S. G o´ mez-Quero, N. Perret and M. A.
Keane, Catal. Sci. Technol., 2011, 1, 652.
5 F. C ´a rdenas-Lizana, S. G o´ mez-Quero and M. A. Keane,
Appl. Catal., A, 2008, 334, 199.
6 F. C ´a rdenas-Lizana, S. G ´o mez-Quero, C. J. Baddeley and
M. A. Keane, Appl. Catal., A, 2010, 387, 155.
7 B. Coq, A. Tijani and F. Figu ´e ras, J. Mol. Catal., 1991,
Selective hydrogenation of o-CNB
The liquid phase catalytic hydrogenation was carried out in a
00 mL stainless steel autoclave, which was charged with o-CNB
3.17 mmol), the catalyst (0.1 g) and ethanol solvent (50 mL).
At first, air was flushed out of the reactor with nitrogen at room
temperature. Then the reactor was placed into an oil bath which
was preset to the certain reaction temperature for 2 h under
1
(
68, 331.
1
1
8 B. Coq, A. Tijani and F. Figu ´e ras, J. Mol. Catal., 1992, 71, 317.
9 B. Coq and F. Figu ´e ras, Coord. Chem. Rev., 1998,
1
78–180, 1753.
0 Y.-C. Liu, C.-Y. Huang and Y.-W. Chen, Ind. Eng. Chem. Res.,
006, 45, 62.
1 Y.-C. Liu and Y.-W. Chen, Ind. Eng. Chem. Res., 2006,
5, 2973.
2 Z. Yu, S. Liao, Y. Xu, B. Yang and D. Yu, J. Chem. Soc., Chem.
Commun., 1995, 1155.
3 V. Kratky, M. Kralik, M. Mecarova, M. Stolcova, L. Zalibera
and M. Hronec, Appl. Catal., A, 2002, 235, 225.
4 Z. Yu, S. Liao, Y. Xu, B. Yang and D. Yu, J. Mol. Catal. A:
Chem., 1997, 120, 247.
2
atmospheric pressure. Subsequently, H was fed into the reactor
2
2
2
2
2
at the designated pressure and the hydrogenation was initiated by
starting stirring. The total pressure of the autoclave was kept
constant at 2.0 MPa during hydrogenation by supplying hydrogen
continuously from the gas cylinder through the pressure regulator
to compensate for the consumed hydrogen in the closed autoclave.
After reaction, the reactor was cooled with an ice-water bath for
2
4
2
0 min, and depressurized carefully. Finally, the liquid products were
analyzed by an Agilent GC7890A gas chromatograph equipped with
flame ionization detector and HP-5 capillary column. The injector
temperature was 250 1C, and the detector column temperature was
ꢀ
1
25 X.-X. Han, R.-X. Zhou, G.-H. Lai, B.-H. Yue and X.-M. Zheng,
increased from 100 to 150 1C with a ramp rate of 5 1C min
.
J. Mol. Catal. A: Chem., 2004, 209, 83.
2
6 X. X. Han, R. X. Zhou, G. H. Lai, B. H. Yue and X. M. Zheng,
Catal. Lett., 2003, 89, 255.
Acknowledgements
We gratefully acknowledge the financial support from the 27 A. Tijani, B. Coq and F. Figu ´e ras, Appl. Catal., 1991, 76, 255.
73 Program (2011CBA00506) and the National Natural Science 28 B. Coq, A. Tijani, R. Dutartre and F. Figu ´e ras, J. Mol. Catal.,
9
Foundation of China.
1993, 79, 253.
2
3
9 J. Xiong, J. X. Chen and J. Y. Zhang, Catal. Commun., 2007,
8, 345.
Notes and references
0 X. D. Wang, M. H. Liang, J. L. Zhang and Y. Wang, Curr. Org.
Chem., 2007, 11, 299.
1
G. K o¨ nnecker, A. Boehncke and S. Schmidt, Fresenius
Environ. Bull., 2003, 12, 589.
31 P. S. Braterman, Z. P. Xu and F. Yarberry, in Handbook of
Layered Materials, ed. S. M. Auerbach, K. A. Carrado and
P. K. Dutta, New York, 2004.
2
Y. Y. Chen, C. A. Wang, H. Y. Liu, J. S. Qiu and X. H. Bao,
Chem. Commun., 2005, 5298.
9
90 Catal. Sci. Technol., 2013, 3, 982--991
This journal is c The Royal Society of Chemistry 2013

View Article Online
Paper
Catalysis Science & Technology
3
2 M. R. Othman, Z. Helwani, Martunus and W. J. N. 52 M. Del Arco, P. Malet, R. Trujillano and V. Rives, Chem.
Fernando, Appl. Organomet. Chem., 2009, 23, 335. Mater., 1999, 11, 624.
3 X. Guo, F. Zhang, D. G. Evans and X. Duan, Chem. Commun., 53 E. Heracleous, A. F. Lee, K. Wilson and A. A. Lemonidou,
010, 46, 5197. J. Catal., 2005, 231, 159.
4 D. G. Evans and R. C. T. Slade, Struct. Bonding, 2006, 119, 1. 54 Y. Cesteros, P. Salagre, F. Medina and J. E. Sueiras, Appl.
5 F. Li and X. Duan, Struct. Bonding, 2006, 119, 193. Catal., B, 2000, 25, 213.
6 A. I. Tsyganok, T. Tsunoda, S. Hamakawa, K. Suzuki, 55 J. L. Chen, Y. D. Li, Z. Q. Li and X. X. Zhang, Appl. Catal., A,
K. Takehira and T. Hayakawa, J. Catal., 2003, 213, 191. 2004, 269, 179.
7 C. G ´e rardin, D. Kostadinova, N. Sanson, B. Coq and 56 C. A. Bessel, K. Laubernds, N. M. Rodriguez and R. T. K.
D. Tichit, Chem. Mater., 2005, 17, 6473. Baker, J. Phys. Chem. B, 2001, 105, 1115.
8 C. G ´e rardin, D. Kostadinova, B. Coq and D. Tichit, Chem. 57 A. E. Martell, R. M. Smith and R. J. Motekaitis, NIST critically
3
2
3
3
3
3
3
3
Mater., 2008, 20, 2086.
selected stability constants of metal complexes database,
9 C. Resini, T. Montanari, L. Barattini, G. Ramis, G. Busca,
U.S. Department of Commerce, Gaithersburg, 2004.
S. Presto, P. Riani, R. Marazza, M. Sisani, F. Marmottini and 58 Y. Tang, D. Yang, F. Qin, J. Hu, C. Wang and H. Xu, J. Solid
U. Costantino, Appl. Catal., A, 2009, 355, 83. State Chem., 2009, 182, 2279.
0 A. Olafsen, Å. Slagtern, I. M. Dahl, U. Olsbye, Y. Schuurman 59 N. Yan, Y. Yuan and P. J. Dyson, Chem. Commun., 2011,
and C. Mirodatos, J. Catal., 2005, 229, 163. 47, 2529.
1 Z. Yu, D. Chen, M. Rønning, T. Vrålstad, E. Ochoa-Fern ´a ndez 60 Y. Yuan, N. Yan and P. J. Dyson, Inorg. Chem., 2011, 50, 11069.
4
4
4
4
4
4
4
and A. Holmen, Appl. Catal., A, 2008, 338, 136.
2 N. Barrabes, D. Cornado, K. Foettinger, A. Dafinov, J. Llorca,
F. Medina and G. Rupprechter, J. Catal., 2009, 263, 239.
3 J. T. Feng, Y. J. Lin, D. G. Evans, X. Duan and D. Q. Li,
J. Catal., 2009, 266, 351.
4 S. Takenaka, T. Arikea, H. Matsune, E. Tanabeb and
M. Kishida, J. Catal., 2008, 257, 345.
5 M. Garc ´ı a-Di ´e guez, I. S. Pieta, M. C. Herrera, M. A. Larrubia
and L. J. Alemany, J. Catal., 2010, 270, 136.
61 J. Liu, M. R. Zubiri, B. Vigolo, M. Dossot, Y. Fort,
J. J. Ehrhardt and E. McRae, Carbon, 2007, 45, 885.
62 M. Wei, X. Y. Xu, X. R. Wang, F. Li, H. Zhang, Y. L. Lu, M. Pu,
D. G. Evans and X. Duan, Eur. J. Inorg. Chem., 2006, 2831.
63 Y. F. Gao, M. Nagai, Y. Masuda, F. Sato, W. S. Seo and
K. Koumoto, Langmuir, 2006, 22, 3521.
64 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou,
R. A. Pierotti, J. Rouqu ´e rol and T. Siemieniewska, Pure Appl.
Chem., 1985, 57, 603.
6 K. K. Kim, J. J. Bae, H. K. Park, S. M. Kim, H. Z. Geng, 65 B. W. Hoffer, E. Crezee, F. Devred, P. R. M. Mooijman,
K. A. Park, H. J. Shin, S. M. Yoon, A. Benayad, J. Y. Choi and
W. G. Sloof, P. J. Kooyman, A. D. van Langeveld, F. Kapteijn
Y. H. Lee, J. Am. Chem. Soc., 2008, 130, 12757.
and J. A. Moulijn, Appl. Catal., A, 2003, 253, 437.
4
4
4
7 W. Deng, M. Liu, X. Tan, Q. Zhang and Y. Wang, J. Catal., 66 J. Li, S. B. Tang, L. Lu and H. C. Zeng, J. Am. Chem. Soc.,
010, 271, 22. 2007, 129, 9401.
8 Y. S. Chun, J. Y. Shin, C. E. Song and S. G. Lee, Chem. 67 N. Keller, G. Koehl, B. Pietruszka and V. Keller, Catal. Today,
Commun., 2008, 942. 2005, 102–103, 266.
9 X. B. Fu, H. Yu, F. Peng, H. J. Wang and Y. Qian, Appl. Catal., 68 U. Roland, R. Salzer, T. Braunschweig, F. Roessner and
2
A, 2007, 321, 190.
0 D. Eder and A. H. Windle, Adv. Mater., 2008, 20, 1787.
1 H. Wang, X. Xiang and F. Li, AIChE J., 2010, 56, 768.
H. Winkler, J. Chem. Soc., Faraday Trans., 1995, 91, 1091.
69 V. Kratky, M. Kralik, M. Mecarova, M. Stolcova, L. Zalibera
and M. Hronec, Appl. Catal., A, 2002, 235, 225.
5
5
This journal is c The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 982--991 991