The promoted catalytic hydrogenation performance of bimetallic Ni-Co-B noncrystalline alloy nanotubes

Article abstract of DOI:10.1039/c9ra05540a

A noncrystalline Ni-B alloy in the shape of nanotubes has demonstrated its superior catalytic performance for some hydrogenation reactions. Remarkable synergistic effects have been observed in many reactions when bimetallic catalysts were used; however, bimetallic noncrystalline alloy nanotubes are far less investigated. Here, we report a simple acetone-assisted lamellar liquid crystal approach for synthesizing a series of bimetallic Ni-Co-B nanotubes and investigate their catalytic performances. The dilution effect of acetone on liquid crystals was characterized by small-angle X-ray diffraction (SAXRD) and scanning electron microscopy (SEM). The Ni/Co molar ratio of the catalyst was varied to study the composition, porous structure, electronic interaction, and catalytic efficiency. In the liquid-phase hydrogenation of p-chloronitrobenzene, the as-prepared noncrystalline alloy Ni-Co-B nanotubes exhibited higher catalytic activity and increased stability as compared to Ni-B and Co-B alloy nanotubes due to electronic interactions between the nickel and cobalt. The excellent hydrogenation performance of the Ni-Co-B nanotubes was attributed to their high specific surface area and the characteristic confinement effects, compared with Ni-Co-B nanoparticles.

Full text of DOI:10.1039/c9ra05540a

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PAPER
The promoted catalytic hydrogenation performance
of bimetallic Ni–Co–B noncrystalline alloy
nanotubes†
Cite this: RSC Adv., 2019, 9, 26456
ab
a
a
a
and Xuefeng Guo *a
Min Mo, Mingjiang Xie, Xiaojuan Guo, Weiping Ding
A noncrystalline Ni–B alloy in the shape of nanotubes has demonstrated its superior catalytic performance
for some hydrogenation reactions. Remarkable synergistic effects have been observed in many reactions
when bimetallic catalysts were used; however, bimetallic noncrystalline alloy nanotubes are far less
investigated. Here, we report a simple acetone-assisted lamellar liquid crystal approach for synthesizing
a series of bimetallic Ni–Co–B nanotubes and investigate their catalytic performances. The dilution
effect of acetone on liquid crystals was characterized by small-angle X-ray diffraction (SAXRD) and
scanning electron microscopy (SEM). The Ni/Co molar ratio of the catalyst was varied to study the
composition, porous structure, electronic interaction, and catalytic efficiency. In the liquid-phase
hydrogenation of p-chloronitrobenzene, the as-prepared noncrystalline alloy Ni–Co–B nanotubes
exhibited higher catalytic activity and increased stability as compared to Ni–B and Co–B alloy nanotubes
due to electronic interactions between the nickel and cobalt. The excellent hydrogenation performance
of the Ni–Co–B nanotubes was attributed to their high specific surface area and the characteristic
confinement effects, compared with Ni–Co–B nanoparticles.
Received 18th July 2019
Accepted 15th August 2019
DOI: 10.1039/c9ra05540a
rsc.li/rsc-advances
to their unique long-range disordering with short-range
Introduction
9–12
ordering structure.
Due to their isotropic structure, it is
Scientists are increasing their efforts to synthesize catalysts with difficult to prepare noncrystalline alloy catalysts with NT
controlled structure and morphology due to the benets morphology.
observed from the properties of natural materials when they are
enhanced and the remarkable functions that result from their Co–B, Fe–B, and Ni–Cu–B) can be synthesized by a lamellar
We rst reported that noncrystalline alloy NTs (e.g., Ni–B,
1–3
unique shape and/or morphology. When used as catalysts, liquid crystal (LC) route, which exhibited high activity and
13–15
nanotubular (NT) materials present some advantages over superior selectivity in hydrogenation reactions.
The lamellar
nanoparticles (NPs) in terms of increased surface area, low LC is a nematic phase, and can be considered as a solid solu-
16
density, easy recovery, self-supporting capacity, and surface tion. This property is in favour of the formation of non-
4
permeability. In particular, the inner surface of the nanotubes crystalline alloy NTs because of the rapid and strongly
(
NTs) is concave, and may exhibit enhanced catalytic properties exothermic reduction of metallic ions by borohydride. However,
relative to the exposed convex surfaces of the NPs. Their inner in previous reports, some NPs generally coexisted with non-
cavity could also function as nanoreactors to improve the crystalline alloy NTs, which affected the performance of the NT
5
13,15
reaction efficiency by remarkable connement effects.
Although almost all of the reported studies have found that via the LC route, it still remains a challenge to obtain high-yield
catalysts.
Although noncrystalline alloy NTs can be obtained
4,5
the catalytic activity of CNT-supported catalysts is enhanced,
there has recently been an upsurge in the use of non-carbon
support catalysts in the development of NT morphology.
noncrystalline alloy NTs with less NP counterparts.
Hydrogenation of aromatic halonitro compounds (e.g., p-
chloronitrobenzene (p-CNB)) to the corresponding haloamine
6
–8
Noncrystalline alloys (e.g., Ni–B) are new powerful catalysts due (e.g., p-chloroaniline (p-CAN)) is of great industrial importance,
not only because aromatic haloamines are extensively used as
chemical intermediates, but also because this procedure
a
Key Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, prevents environmental pollution and is a satisfactory solution
Nanjing University, Nanjing 210093, China. E-mail: guoxf@nju.edu.cn; Fax: +86 25
17–19
that is practiced industrially on a large scale.
Noncrystalline
8
b
3686251; Tel: +86 25 83595077
alloys have been reported to show excellent catalytic perfor-
Department of Educational Science, Hunan First Normal University, Changsha
mance in the liquid phase hydrogenation of p-CNB to p-CAN for
4
10205, China
20,21
many years.
Adding specic modiers (e.g., Pd, Co, Cu, Mo,
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra05540a
and W) to Ni–B noncrystalline alloy during the preparation is an
1
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efficient strategy to improve the catalytic behaviours of non- spectrometry was performed with a J-A1100 spectrometer. The
14,21
crystalline alloy catalysts.
These promoter metals efficiently nitrogen sorption isotherms were measured at liquid nitrogen
and signicantly increased the activity, selectivity, and stability temperature using an ASAP 2020 apparatus. H2 temperature
18
of catalysts. The second metal was expected not only to act as programmed desorption (H -TPD) measurements were ob-
2
an electron donor ligand, thus increasing the electron density tained by using a Micromeritics AutoChem TP-5080 apparatus
on the active metal atoms that favours the reaction kinetics, but with a thermal conductivity detector. The samples were pre-
also to endow new capabilities due to a synergistic effect treated at 473 K for 30 min in a He stream at a ow rate of 30
2
0
ꢀ1
between the two metals.
mL min and were cooled to 373 K, followed by exposure to
/Ar stream for 30 min, and then, the sample was purged
In the current study, based on the LC route, we further a H
2
developed an acetone-assisted lamellar LC method to greatly with a pure He stream for a certain period of time until
improve the yield and quality of noncrystalline alloy NT cata- a constant baseline was attained. H -TPD was carried out in the
2
ꢀ1
lysts, and have successfully prepared a series of bimetallic Ni– range of 303–973 K at a heating rate of 10 K min . X-ray
Co–B noncrystalline alloy nanotubes. The dilution effect of photoelectron spectroscopy (XPS) was performed with
acetone on liquid crystal was studied in detail. With a proper a Thermo ESCALAB 250 using Al Ka radiation (hn ¼ 1486.6 eV)
+
ratio of Ni/Co, the bimetallic NTs exhibited higher activity and aer etching the surface by Ar ions for 200 s. The spectrometer
better selectivity than Ni–B NTs or the corresponding NPs for was operated at 20 eV pass energy. All the binding energy (BE)
the catalytic hydrogenation of p-CNB to p-CAN. Remarkably, the values were calibrated using C1s ¼ 285.0 eV as a reference.
obtained bimetallic noncrystalline Ni–Co–B NTs possess
Hydrogenation of p-CNB was used to measure the catalytic
a signicantly more stable morphology than the Ni–B NTs, performance. The 100 mL reactor was charged with 2 mmol Ni
indicating the intriguing synergistic effect between the two catalyst and 2.50 g of p-CNB in 60 mL of absolute ethanol
metals.
solution. When the designated temperature (353 K or 373 K) was
reached, hydrogen was fed to maintain the predetermined
pressure (1.2 MPa) (time zero) throughout the reaction. During
the experiment, samples were periodically withdrawn and
Experimental
All chemicals were of analytical grade and were used as received analyzed with a GC-2014 Shimadzu gas chromatograph.
without further purication. In a typical synthesis, the Ni–Co–B
NTs were prepared as follows: NiCl
6H
with (1S)-(+)-10-camphorsulfonic acid (CSA, 0.005 mol, 1.16 g) LC materials can self-organize into various nanostructured
2
$6H
2
O (x mol) and CoCl
2
-
Results and discussion
$
2
O (y mol) (x + y ¼ 0.005 mol) were dissolved in H
2
O (9 mL)
and Tween 60 (0.005 mol, 6.15 g) at 333 K. The mixture was then phases, such as layered, columnar, micellar, and bi-continuous
16
cooled to approximately 298 K. Acetone (5, 10, 15 mL) was added cubic structures. Therefore, it is very important that the
to the above mixture, which was then agitated for 15 min. A precursor possesses a lamellar structure, i.e., the precursor in
mixed solution of 4 M NaBH
pumped into the above homogeneous mixture, and the resul- current synthesis of ternary noncrystalline alloy NTs. The
tant mixture was maintained for 48 h under an N atmosphere. precursor of the LC composite with the metal source was coated
4
and 0.1 M NaOH was continuously this case is the LC template and metal sources used in the
2
The obtained solid was collected and washed with distilled on a glass substrate, and XRD at 298 K (Fig. 1a) indicated that
ethanol and water, and then dried in owing nitrogen. The as- there were strong diffraction peaks in the small angle region of
prepared noncrystalline alloy NTs were denoted as Ni–B NTs XRD, which can be indexed as the (001) and (002) reections for
(
without Co source), Ni–Co–B NTs-1 (with 1 : 0.1 : 4 of the lamellar phase with a repeat distance of d ¼ 7.9 nm. The
ꢁ
Ni : Co : B ratio in the starting solution), Ni–Co–B NTs-2 peak at 2q ¼ 5.1 is due to the precipitate of camphorsulfonic
(
1 : 0.3 : 4 of Ni : Co : B), Ni–Co–B NTs-3 (1 : 0.5 : 4 of acid (CSA). The lamellar LC system can be considered as a solid
Ni : Co : B), Ni–Co–B NTs-4 (1 : 1 : 4 of Ni : Co : B), Ni–Co–B solution and is favourable for preparing noncrystalline alloy
NTs-5 (1 : 2 : 4 of Ni : Co : B), and Co–B NTs (without Ni source). NTs with large surface areas. However, the drawback of the
The corresponding synthetic parameters are presented in Table method we previously reported is that the LC is too viscose to be
1
1
. The Ni–Co–B nanoparticles (named as Ni–Co–B NPs with easily agitated.
: 0.1 : 4 of Ni : Co : B) were prepared with only CSA (without In the current synthesis, acetone was introduced to improve
the uid property of the LC precursor without altering its
Tween 60) in 45 mL aqueous solution.
Powder X-ray diffraction (XRD) measurements were per- lamellar structure, which divides the LC system into domains,
formed with a Philips X'Pert Pro MPD X-ray diffractometer with and correspondent XRD and SEM results are shown in Fig. 1a
graphite-monochromated high-intensity CuKa radiation at 40 and 2a–d respectively. When the LC precursor was diluted with
kV and 30 mA. Transmission electron microscopy (TEM) and 5, 10, and 15 mL of acetone, the LC still retained the lamellar
selected-area electron diffraction (SAED) images were recorded structure. Fig. 2 shows that the dilution with 10 mL acetone
with a JEM-100S electron microscope (JEOL) at an acceleration (Fig. 2c) resulted in LC domains of suitable sizes that can
voltage of 80 kV. Scanning electron microscopy (SEM) images sufficiently react with borohydride and generate NTs. The
were taken with a S4800 electron microscope operating at 1.0 essential physical and chemical properties of all the samples
kV, and a gold thin lm was coated on the sample before the obtained in the current investigation are summarized in Table
characterization. The inductively coupled plasma (ICP) 1.
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Table 1 Composition, surface area, pore size, pore volume, H
2
uptake, and reaction rate constants of the catalysts
b
ꢀ1
K (min
)
Composition
(ICP results)
Pore
Pore volume
H
2
uptake
a
2
ꢀ1
3
ꢀ1
ꢀ1
353 K (ꢂ10ꢀ2
373 K (ꢂ10ꢀ2
Sample name
Catalysts
S
BET (m g
)
size (nm) (cm g
)
(mmol g
)
)
)
Ni–B NTs
NiB (1 : 4)
Ni72.2
B
27.8
115.6
114.8
113.7
71.2
62.1
63.2
64.7
65.1
67.0
67.3
—
0.73
0.73
0.73
0.73
0.73
0.72
0.72
—
80.1
83.9
83.0
82.5
81.7
80.6
78.4
24.7
1.30
2.19
2.12
1.87
1.39
0.62
0.12
1.23
11.6
23.0
20.4
15.0
9.35
2.06
0.30
3.76
Ni–Co–B NTs-1 NiCoB (1 : 0.1 : 4) Ni66.2Co4.2
Ni–Co–B NTs-2 NiCoB (1 : 0.3 : 4) Ni52.0Co8.7
Ni–Co–B NTs-3 NiCoB (1 : 0.5 : 4) Ni45.8Co13.2
Ni–Co–B NTs-4 NiCoB (1 : 1 : 4)
Ni–Co–B NTs-5 NiCoB (1 : 2 : 4)
B
B
B
B
B
29.6
39.3
41.0 113.4
42.5 110.2
46.1 109.8
107.4
Ni30.4Co27.1
Ni19.5Co34.4
Co66.9B
33.1
Co–B NTs
Ni–Co–B NPs
CoB (1 : 4)
NiCoB (1 : 0.1 : 4) Ni65.4Co4.4
B
30.2
33.6
a
b
The catalysts are denoted as NiCoB (x : y : z), where x : y : z is the atomic ratio of Ni : Co : B in the starting solution. K is the reaction rate constant
calculated using rst order reaction kinetics.
The XRD patterns of Ni–Co–B NTs with varied Co content are method used. Fig. 3b shows the Ni–Co–B NTs-1 prepared by
presented in Fig. 1b. A single broad peak at approximately 2q ¼ diluting the LC precursor with acetone. The NTs exhibit good
ꢁ
4
5 in each pattern indicates that the as-prepared samples are uniformity with a length up to several millimeters, due to the
in noncrystalline phase, and no distinct peak corresponding to adequate multiple interactions among the LC precursor diluted
ꢀ
2+
2+
any crystalline phases of nickel, cobalt, or boron is detected. It with acetone and the easy contact of BH4 with Ni and Co .
has been demonstrated that the Ni–B alloy NTs with a non- The release of hydrogen loosens and peels the nanosheets away
crystalline structure are benecial for hydrogenation reactions from the lamellar template more easily and uniformly. The
because the noncrystalline alloy possesses a large number of quality and yield of the NTs can be promoted under these
coordinative unsaturated surface sites. By the addition of conditions, and the amount of NPs decreases, as conrmed by
a second metal (Co), the ternary alloy maintains enhanced TEM observations that indicate that more than 95% of the
stability of its noncrystalline structure, which is important for product is noncrystalline NTs. Fig. 3c shows the TEM image of
22
catalytic applications. It appears that the Ni–Co–B NTs are all Ni–B NTs, which are 60–65 nm in diameter and several microns
13–15
noncrystalline structures with a full range of Ni/Co ratios.
in length, as we previously reported.
Fig. 3d shows the TEM
Fig. 3 shows the TEM images of Ni–Co–B NTs-1, in image of Co–B NTs, which are 70–75 nm in diameter and several
comparison with Ni–B NTs, Co–B NTs, and Ni–Co–B NPs. The microns in length. Fig. 3e shows a TEM image of Ni–Co–B NPs.
Ni–Co–B NTs-1 prepared by the LC precursor without acetone The continuous broad halo rings (insets of Fig. 3a–e) from the
dilution usually have poor uniformity and morphology, while SAED further support the noncrystalline nature of the NTs and
NPs are always present (Fig. 3a). Noncrystalline NTs are the NPs.
major product, with an estimated percentage of more than 60%
The N sorption isotherms and pore size distributions of the
2
according to microscopic observation of the samples. The Ni–Co–B NTs-1 and Ni–Co–B NP samples are presented in
presence of the NPs is presumably due to the weak interactions Fig. S1.† All of the noncrystalline alloys show type IV adsorp-
between the metal ions and surfactants in the so-template tion–desorption isotherms. The presence of hysteresis loops
Fig. 1 (a) Small-angle XRD patterns of the precursor of LCs coated on a glass substrate diluted by 0, 5, 10, and 15 mL of acetone, respectively; (b)
XRD patterns of the Ni–Co–B, Ni–B, Co–B NT, and Ni–Co–B NP catalysts.
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Fig. 2 SEM images of the LCs coated on a silicon substrate diluted by (a) 0 mL, (b) 5 mL, (c) 10 mL, and (d) 15 mL of acetone.
indicates that the samples are mesoporous. The pore size mixture inuenced the composition, pore size, and pore volume
distribution curves display a narrow pore-diameter range for the of the obtained NTs. The content of B in all Ni–Co–B non-
23
NTs.
crystalline alloy samples was apparently higher than that of the
The composition and porous characteristics of the non- Ni–B NTs with the same Ni/B ratios in the starting materials.
crystalline alloy catalysts are summarized in Table 1. In this With the increase in the Co content in the tri-component non-
2
+
study, the amount of the second metal (Co ) in the starting crystalline alloy NTs, the pore size increases, while the surface
Fig. 3 TEM images of the catalysts obtained in the current investigation: (a) Ni–Co–B NTs-1 without precursor dilution during synthesis, (b) Ni–
Co–B NTs-1, (c) Ni–B NTs, (d) Co–B NTs, and (e) Ni–Co–B NPs.
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25
area becomes smaller. It should be noted that all the NTs reaction, and that Co acted as a promoter. The hydrogenation
showed a much higher surface area, e.g., the Ni–Co–B NTs-1 reaction of p-CNB can be treated as a quasi rst order reaction
2
ꢀ1
sample has a surface area of 114.8 m
g , which is larger with respect to p-CNB, by maintaining the hydrogen pressure as
than the surface area of the Ni–Co–B NPs with similar compo- constant during the reaction. The plots of the reaction progress
2
ꢀ1
sition (only 33.6 m g , Table 1).
with reaction time on the different catalysts at 353 and 373 K are
shown in Fig. 5b and d, respectively. The values of reaction rate
constant K, i.e., the slope of the plot, and the selectivity are
listed in Tables 1 and 2, respectively. The sample Ni–Co–B NTs-
Electron states of the samples measured by XPS
Several samples, Ni–B NTs, Ni–Co–B NTs-1, and Ni–Co–B NPs,
were selected for XPS investigation, and their XPS spectra are
shown in Fig. 4. All the samples were etched with Ar for 200
seconds before the collection of the spectra of electron binding
energy. As illustrated in Fig. 4a–b, two peaks at 856.2 and
52.7 eV are respectively assigned to the Ni 2p3/2 level for
oxidized and elemental nickel in Ni–B NTs. In the B1s region,
two XPS peaks with binding energies (BEs) of 188.2 and 192.4 eV
are observed and assigned to elemental and oxidized boron,
1, with a small amount of Co doped, has the highest reaction
rate but relatively lower selectivity for p-CAN. Thereaer, the
activity of the catalysts decreases with the increase in the
amount of Co doped. At 353 K, the activity of Ni–Co–B NTs-1,
Ni–Co–B NTs-2, Ni–Co–B NTs-3, and Ni–Co–B NTs-4 is higher
than that of Ni–B NTs. With more Co doping, the sample Ni–
Co–B NTs-5 is less active than Ni–B NTs.
+
8
2
0
The temperature has great impact on the performance of the
catalysts in hydrogenation reaction. With the temperature
increased, the selectivity of p-CAN decreases. At 373 K, the
activity of Ni–Co–B NTs-1, Ni–Co–B NTs-2, and Ni–Co–B NTs-3 is
higher than that of Ni–B NTs. However, the samples Ni–Co–B
NTs-4 and Ni–Co–B NTs-5 are less active than Ni–B NTs. The
sample Ni–Co–B NPs-6 with cobalt content similar to that of Ni–
Co–B NTs-1 is lowest in activity for the reaction at 353 K and 373
K. The introduction of Co to Ni–B NTs results in both positive
and negative effects on the hydrogenation activities. The posi-
tive effect relates to the synergistic interaction between Ni and
Co in the noncrystalline alloy Ni–Co–B NTs. The effects of Co
may be attributed to its dispersion of Ni metal, resulting in the
formation of additional surface Ni centers. The Ni 2p3/2 binding
energy shied to lower values upon Co doping, implying that
there was an enriched electron density around Ni, and this is in
agreement with the increase in the hydrogenation activities of
2
2
respectively, on the surface of the sample. Two XPS peaks in
Fig. 4c are observed for the Co 2p3/2 level with the BEs near 777.9
and 781.2 eV, which are attributed to elemental and oxidized
2
4
cobalt, respectively. Upon doping Co into the Ni–B NTs, the BE
values of elemental Ni shied to lower ones for Ni–Co–B NTs-1.
The negative shis are evidence that electronic enrichment
occurred around Ni in the alloy NTs. The surface composition of
the samples was calculated by analysis of the relative XPS peak
areas. It was determined that doping Co in Ni–B NTs increased
the electron density around Ni in a concentration-dependent
manner according to the level of dopant.
The ability of the as-prepared catalysts to hydrogenate p-CNB
was tested, with the selectivity for p-CAN upon the doping of Co
being taken into account, and their activities are always very
high. Fig. 5a and c show the time-conversion curves of p-CNB
hydrogenation on the catalysts doped with Co. The hydroge-
nation activity of Ni–B noncrystalline alloy NTs is much higher
than that of Co–B NTs. The latter is almost low for the reaction,
implying that Ni could be viewed as the main active sites for the
23
Ni–Co–B NTs-1. With a larger amount of Co doping, however,
the catalysts become less active than the original Ni–B NTs due
to the decrease in the surface concentration of Ni active sites.
+
Fig. 4 XPS spectra of Ni–Co–B NTs-1, Ni–Co–B NPs, and Ni–B NTs, etched by Ar for 200 s before measurement: (a) Ni 2p3/2, (b) B1s, and (c) Co
3/2
2p .
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Fig. 5 Time-conversion curves of p-CNB hydrogenation on Ni–Co–B NTs and NPs, Ni–B NTs, and Co–B NT catalysts at (a) 353 K and (c) 373 K;
plot of ꢀln(1 ꢀ X) versus time to confirm the first-order reaction type, with X being the conversion of p-CNB at (b) 353 K and (d) 373 K.
Table 2 The selectivity of all catalysts for p-CAN, AN, and NB
Selectivity (%) 353 K Selectivity (%) 373 K
benecial for the catalytic performances. The inner space of the
NTs has a structure-dened effect on the molecules inside the
tubes, which may contribute to the improved catalytic
4,14,26
activity.
Sample name
p-CAN
AN
NB
p-CAN
AN
NB
Fig. 6 shows the H -TPD curves of all catalyst samples. The
2
Ni–B NTs
96
96
96
98
99
99
99
70
3
3
3
2
1
1
1
18
1
1
1
0
0
0
0
12
88
90
91
91
92
93
96
55
9
7
6
6
6
5
3
27
3
3
3
3
2
2
1
18
Ni–Co–B NPs exhibit multiple TPD peaks, indicating the multi-
active sites possessed by the catalysts. The TPD peaks appear at
approximately 436, 479, 590, and 667 K. The two peaks at 436
and 479 are due to weak adsorption of hydrogen and would be
less active than the strongly adsorbed hydrogen atoms that are
Ni–Co–B NTs-1
Ni–Co–B NTs-2
Ni–Co–B NTs-3
Ni–Co–B NTs-4
Ni–Co–B NTs-5
Co–B NTs
associated with the TPD peaks at approximately 590 and 667
Ni–Co–B NPs
27,28
K.
However, only two main TPD peaks are observed for the
nanotubular samples, such as Ni–Co–B NTs, Ni–B NTs, and Co–
B NTs, suggesting that the NTs obtained with this preparation
method contain relatively uniform active sites. Two TPD peaks
over Co–B NTs are observed at temperatures lower than those
from Ni–B NTs, indicating that the Ni–B NTs adsorb hydrogen
more strongly than the Co–B NTs. From Ni–Co–B NTs-1 to Ni–
Co–B NTs-4 catalysts, the principal TPD peak moves gradually to
higher temperatures with the increase in the Co concentration,
showing stronger interaction between the catalysts and
hydrogen upon the formation of tri-component noncrystalline
alloy NTs. The Ni–Co–B NTs-5 sample shows abnormal prop-
erties for hydrogen desorption, which appears at lower
The hydrogenation activity of Ni–Co–B NTs-1 is much higher
than that of Ni–Co–B NPs, as shown in Fig. 5. The composition
of Ni–Co–B NPs is similar to that of Ni–Co–B NTs-1, but the
2
ꢀ1
surface area of Ni–Co–B NPs (33.6 m g ) is less than that of
2
ꢀ1
Ni–Co–B NTs-1 (114.8 m
g , Table 1). The nanotubular
morphology provides a much higher specic surface area of the
catalysts than other shapes, such as NPs. The enhancement in
catalytic activity by the NTs is not merely attributed to the larger
surface area. In addition to the larger surface area of the NTs,
the nanotubular structure of the catalyst would also be
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3
ꢀ1
3
ꢀ1
surface area decreased from 114.8 cm g to 96.5 cm g aer
four consecutive runs, although the NTs reserved their non-
crystalline structure, according to microscopic observation and
SAED.
The possible reaction pathways for p-CNB hydrogenation are
shown in Scheme 1. Hydrogenation of p-CNB follows two paths
in the reaction. One is the hydrogenation of p-CNB to the cor-
responding p-chloroaniline (p-CAN), and the other is the
formation of nitrobenzene (NB) due to the dehalogenation
reaction. Both p-CAN and NB can be hydrogenated further to
produce aniline (AN). Because the –NO group is more electro-
2
negative than –Cl, –NO will occupy the electron-rich Ni active
2
2
site at the start of the reaction. Therefore, the –NO group
adsorbed on the catalyst surface is hydrogenated to form p-CAN.
When –Cl pre-occupies the catalyst surface, hydrogenation fol-
lowed by dehalogenation occurs, leading to the formation of
NB. The greater the electron density around the Ni active site,
23
the higher the hydrogenation activity of the nitro group.
Fig. 6
NPs.
2
H -TPD curves of Ni–B, Co–B, Ni–Co–B NTs, and Ni–Co–B
In this study, the Ni–Co–B NTs-1 catalyst has the highest
electron density, and therefore, it has the highest activity for
hydrogenation of the nitro group and dehalogenation reaction.
The Co-dopant could weaken the extent of electron donation
from the Ni atoms to the aromatic ring in p-CAN, which would
further suppress the hydrodechlorination of p-CAN.
temperatures than that of Ni–B NTs. The H2 uptakes of all
catalysts obtained from the H -TPD data are also listed in Table
2
1. The NT samples have higher H
2
uptakes than those of the
corresponding NP samples, which is attributed to the difference
4,14,15
in surface area and the NT structures among these samples.
Fig. 7 shows the reuse results of Ni–Co–B NTs-1 and Ni–B NT
catalysts. Ni–B NTs display a loss of activity of 15% and 55% at
the third and the fourth times of reuse, respectively (Fig. 7a). Ni–
Co–B NTs-1 exhibits a more robust reuse performance, which
may be attributed to the more stable noncrystalline structure
caused by the small amount of Co doping. The doping of Co
into Ni–B NTs increases their adsorption for hydrogen and
further diminishes the poisoning effect from the adsorption of
24,28
p-CNB.
Fig. 7b and c show the TEM images and the corre-
sponding SAED data for the Ni–Co–B NTs-1 and Ni–B NTs aer
four cycles of a catalytic test. The tubular morphologies of the
NTs remain basically unchanged. The decreases in the activity
of the NTs during reuses may be due to the partial collapse of
the NTs or oxidation during operation. For Ni–Co–B NTs-1,
Brunauer–Emmett–Teller (BET) analysis reveals that the
Scheme 1 Possible reaction pathways for p-CNB hydrogenation.
Fig. 7 (a) p-CNB conversion on Ni–Co–B NTs-1 and Ni–B NTs within 60 min as a function of recycling run number. TEM images of (b) Ni–Co–B
NTs and (c) Ni–B NTs after four catalytic cycles (the insets show the corresponding SAED patterns).
26462 | RSC Adv., 2019, 9, 26456–26463
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Paper
RSC Advances
5
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Acknowledgements
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