Ultrafine NiMoOx nanoparticles confined in mesoporous carbon for the reduction of nitroarenes: effect of the composition and accessibility of the active sites

Article abstract of DOI:10.1039/c8ra09026b

The design of ultrafine NiMoO_x nanoparticles (NPs) confined in hierarchically porous carbon remains a great challenge due to its high calcination temperature. In addition, the composition of active sites of NiMoO_x NPs for the hydrogenation reaction is still ambiguous. Herein, we report a general approach for the synthesis of ultrafine NiMoO_x NPs confined in mesoporous carbon with different morphologies and compositions using the replication method with SBA-15 as a hard template. The pore structure of mesoporous carbon and the Ni/Mo composition valence-state were discovered to be the main factors in the reduction of nitroarenes. The NiMoO_x/mesoporous carbon-platelet (NiMoO_x/MC-PL) with short mesochannels (～350 nm) and high surface area (～995 m² g^?1) possessed excellent catalytic activity towards the reduction of 4-nitrophenol, whereas NiMoO_x/mesoporous carbon-hexagonal-prism (NiMoO_x/MC-HP), NiMoO_x/mesoporous carbon-long-rod (NiMoO_x/MC-LR), and NiMoO_x/mesoporous carbon-spherical (NiMoO_x/MC-SP) with long mesochannels and relatively less surface area exhibited poor catalytic performance. The bifunctional mechanism or electronic synergistic effects of Ni and Mo species enhanced their catalytic performance. A good balance between MoO_x and metallic Ni (NiMoO_x/MC-PL-450) was found to be suitable for the reduction of 4-NP.

Full text of DOI:10.1039/c8ra09026b

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Ultrafine NiMoOx nanoparticles confined in
mesoporous carbon for the reduction of
nitroarenes: effect of the composition and
accessibility of the active sites†
Cite this: RSC Adv., 2019, 9, 4571
*
Daowei Gao
Shuna Li, Yipin Lv, Guolong Song, Cuncheng Li,
*
and Guozhu Chen
The design of ultrafine NiMoOx nanoparticles (NPs) confined in hierarchically porous carbon remains a great
challenge due to its high calcination temperature. In addition, the composition of active sites of NiMoOx
NPs for the hydrogenation reaction is still ambiguous. Herein, we report a general approach for the
synthesis of ultrafine NiMoOx NPs confined in mesoporous carbon with different morphologies and
compositions using the replication method with SBA-15 as a hard template. The pore structure of
mesoporous carbon and the Ni/Mo composition valence-state were discovered to be the main factors in
the reduction of nitroarenes. The NiMoOx/mesoporous carbon-platelet (NiMoOx/MC-PL) with short
mesochannels (ꢀ350 nm) and high surface area (ꢀ995 m2
g
ꢁ1) possessed excellent catalytic activity
towards the reduction of 4-nitrophenol, whereas NiMoOx/mesoporous carbon-hexagonal-prism
(NiMoOx/MC-HP), NiMoOx/mesoporous carbon-long-rod (NiMoOx/MC-LR), and NiMoOx/mesoporous
carbon-spherical (NiMoOx/MC-SP) with long mesochannels and relatively less surface area exhibited
poor catalytic performance. The bifunctional mechanism or electronic synergistic effects of Ni and Mo
species enhanced their catalytic performance. A good balance between MoOx and metallic Ni (NiMoOx/
MC-PL-450) was found to be suitable for the reduction of 4-NP.
Received 31st October 2018
Accepted 18th January 2019
DOI: 10.1039/c8ra09026b
rsc.li/rsc-advances
activity, have attracted widespread attention.13,14 Compared to
pure Ni, Ni-based catalysts exhibit a much higher catalytic
1. Introduction
performance owing to the so-called synergistic effect. For
example, Zhao et al. reported that Ni/TiO2–C composites
possessed an excellent catalytic performance owing to the
synergistic effect between Ni and TiO2.15 Xu et al. synthesized Ni/
CeO2ꢁx/Pd by the hydrothermal method. Due to their synergistic
effect, Ni/CeO2ꢁx/Pd manifested a super catalytic activity and
stability for the hydrogenation of styrene and 4-NP.16 The
different components in the Ni-based catalysts also had a great
inuence on the catalytic properties. Lu et al. found that Ni–
MoO3/CN@SBA-15 showed an outstanding catalytic performance
and chemoselectivity towards the reduction of various
substituted nitroarenes compared to those of Ni/CN@SBA-15.17
Although great efforts have been made in the synthesis and
catalysis of Ni-based catalysts, there are still a few issues that we
need to address. First, further stabilization of Ni-based NPs is
a priority because the interaction between Ni-based NPs and
supports is not strong enough. Hence, aggregation and leaching
of Ni-based NPs would result in the signicant loss of catalytic
activity.18,19 Second, the inuence of the support morphology and
components of the Ni-based catalysts on the catalytic perfor-
mance still needs to be explored to fully understand the mech-
anism for the reduction of 4-NP.
Nitroarenes are regarded as the most common pollutants from
chemical, pharmaceutical, and industrial sources.1–4 4-Nitro-
phenol (4-NP), one typical species of nitroarenes, is difficult to
dispose of due to its high stability in water. At present, the most
common methods for treating 4-NP include microbial degrada-
tion, electrochemical treatment, and catalytic reduction.5–7
Among them, the selective reduction of 4-NP to 4-aminophenol
(4-AP) with NaBH4 is of much interest because not only is this
catalytic reaction green and effective, but also 4-AP is an impor-
tant intermediate as a reduction product that can be widely
applied in various elds such as dyes and pharmaceuticals.8,9 It is
generally recognized that precious metals (e.g., Au, Pt, and Pd)
exhibit a superior performance for the reduction of 4-NP.10–12
However, the high cost and shortage of resources would inevi-
tably become an obstacle to the wide application of precious
metals. Therefore, transition metals with low costs and large
storage capacities, especially Ni with a high selectivity and
School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, PR
China. E-mail: chm_gaodw@ujn.edu.cn; chm_chengz@ujn.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra09026b
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Scheme 1 Illustration of the synthesis of NiMoOx/MC-LR. (A) Ordered mesoporous silica SBA-15-LR template. (B) A after the impregnation of Ni
and Mo. (C) B after being calcined at 550 ꢂC for 4 h. (D) C after the impregnation of sucrose and carbonization. (E) Ordered mesoporous NiMoOx/
MC-LR.
To enhance the stability of Ni-based catalysts, various carbon and had a high stability attributed to the connement of
structures were designed to x the Ni-based NPs such as the the mesochannel. The bifunctional mechanism or electronic
core–shell structure (Ni/nanorod-CeO2@SiO2 and NiCo2O4@- synergistic effects of Ni and Mo species for the reduction of 4-NP
MnO2), encapsulation into molecular sieve pores (Ni–ZrO2/ and other nitroarenes with different substituent groups were
MCM-41 and NiO–CuO/SBA-15), and carbon nanotubes systematically investigated. A possible mechanism for the
(NiCo2O4/CNT and Ni–CeO2/CNT).20–25 It is well-known that the reduction of nitroarenes catalyzed by the NiMoOx bimetallic
molecular sieve with a porous structure can not only prevent the catalysts was also proposed.
aggregation and loss of NPs, but the channel can also facilitate
the diffusion of substrate molecules, which is benecial to
2. Experimental
2.1 Materials
catalytic reactions.26 More importantly, with the development of
nanotechnology, researchers could precisely control the
dimensions of molecular sieves (e.g., morphology, pore size,
and pore length), which was absolutely helpful to study the
kinetic process of catalytic reactions that used molecular sieves
as catalyst supports. In addition to mesoporous molecular
sieves, mesoporous carbon (MC) was also widely used in various
catalyst supports because of its good alkali resistance and
electrical conductivity.27 However, mesoporous carbon oen
has difficulty in controlling the pore size distribution and pore
uniformity. When the mesoporous carbon was derived from
mesoporous molecular sieves that acted as templates, the as-
prepared carbon inherited the unique structural characteris-
tics of molecular sieves as well as combined efficiently the
structural advantages of mesoporous molecular sieves and
carbon.
All chemicals were analytical grade reagents and used without
further purication. Sucrose (C12H22O11), sodium hydroxide
(NaOH), sodium borohydride (NaBH4), and ethanol (C2H6O)
were obtained from Sinopharm Chemical Reagent Co. Ltd.
Carbon disulde (CS2) and p-nitrophenol (C6H5NO3) were
purchased from Shanghai Macklin Biochemical Co. Ltd.
Sulfuric acid (H2SO4) was purchased from Beijing Chemical
Reagent Company.
2.2 Synthesis of NiMoOx/MC catalysts with different
morphologies
The mesoporous SBA-15 silica used as the template was
synthesized according to the literature.28 The NiMoOx/SBA-15
catalysts with different morphologies were synthesized by the
impregnation method. Aer impregnation, the samples were
dried at 100 ꢂC for 4 h and then calcined at 550 ꢂC for 4 h. The
procedure is shown in Scheme 1. For MC, a typical synthesis
method was followed: 1.0 g of NiMoOx/SBA-15 was dissolved in
5 mL of water containing1.25 g of sucrose and 0.14 g of H2SO4.
The mixture was stirred for 1 h and dried at 100 ꢂC for 5 h. The
mixture was then dried at 160 ꢂC for 5 h. The obtained powder
was mixed with a solution that included 0.8 g of sucrose and
0.14 g of H2SO4. The mixture was then heated at 100 ꢂC for 5 h
Herein, we report a general synthesis procedure for the
ultrane NiMoOx NPs conned in MC with a tunable pore
structure and NiMo composition valence-state using SBA-15 as
a hard template. First, the NiMoOx/SBA-15 catalysts were
synthesized by co-impregnating Ni and Mo precursors into the
mesochannels of SBA-15 (Scheme 1A–C). Second, NiMoOx/MC
catalysts were prepared by the replication method using
NiMoOx/SBA-15 as a hard template (Scheme 1C–E). The ultra-
ne NiMoOx NPs with diameters less than 2 nm were uniformly
distributed on the mesochannel structure of the mesoporous
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and dried at 160 ꢂC for 5 h. The black powder was calcined oxidation states of the chemical species were characterized
under N2 atmosphere at 800 ꢂC for 2 h. The resulting composite using a X-ray photoelectron spectrometer (XPS) on the PHI5000
was added into a 2 M NaOH solution at room temperature to Versaprobe system. The contents of Ni and Mo were obtained
remove the silica. Aer ltration and washing, all samples were from an inductively coupled plasma mass spectrometer (ICP-
dried in an oven. The obtained catalysts were denoted as MS, Thermo Scientic XSeries-2).
NiMoOx/MC-PL
NiMoOx/MC-SP
(NiMoOx/mesoporous
(NiMoOx/mesoporous
carbon-platelet),
The hydrogen temperature-programmed reduction (H2-TPR)
carbon-spherical), was performed on a PCA-1200 instrument. The NiMoOx/MC-PL
NiMoOx/MC-LR (NiMoOx/mesoporous carbon-long-rod), and catalyst was pretreated under an Ar atmosphere (30 mL minꢁ1
)
ꢂ
NiMoOx/MC-HP
(NiMoOx/mesoporous
carbon-hexagonal- at 100 C for 30 min. The samples were then reduced with 7%
prism). For comparison, catalysts with different compositions H2/Ar mixed gas (30 mL minꢁ1) from 25 C to 700 C.
ꢂ
ꢂ
were prepared by the same method, and were designated as
NiO/MC-PL, MoO3/MC-PL, NiMoOx, and MC-PL. For further
2.4 Catalyst measurements
comparison, catalysts with different Ni/Mo valence-states were
prepared by hydrogen reduction at different temperatures
(450 ꢂC and 750 ꢂC) and CS2 vulcanization (referred to as
NiMoOx/MC-PL-450, NiMoOx/MC-PL-750, and NiMoS/MC-PL,
respectively).
The catalytic reduction of 4-NP over catalysts was carried out at
room temperature, which was analyzed by a UV-Visible spec-
troscopy instrument (TU-1901). Freshly prepared NaBH4
solution (1.25 mL, 0.02 M), a certain amount of catalyst solu-
tion (1.25 mL, 0.4 mg mLꢁ1), and 4-NP (25 mL, 0.012 M) were
mixed together. Subsequently, the absorbance of the solution
in the range of 200 to 500 nm was measured using an UV-Vis
2.3 Catalyst characterization
The X-ray diffraction (XRD) patterns of the catalysts were char- absorption spectrometer. The corresponding reduction equa-
acterized with a powder X-ray diffractometer (Shimadzu XRD tions are as follows:
6000). The surface morphologies of the catalysts were obtained
The conversion of 4-nitrophenol is expressed by
(1)
using an emission scanning electron microscope (SEM-
SU8010). High-resolution transmission electron microscopy
(HRTEM) and elemental mapping were performed with a Tecnai
F20 instrument at 200 kV. The surface area and pore size were
analyzed on a Micromeritics TriStar II 3020 analyzer. The
ꢀ
ꢁ
At
Conversionð4-NPÞ ¼ 1 ꢁ
ꢃ 100%
(2)
A0
where A0 and At are the absorbance of 4-NP at time 0 and t,
respectively.
Fig. 1 XRD patterns for various morphologies of NiMoOx/MC (a and b) in the small-angle and wide-angle.
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ꢁlnðCt=C0Þ
k ¼
(3)
(4)
t
k
K ¼ ꢁ
m
where k is the rate constant, C0 and Ct are 4-NP concentrations
at time 0 and t, respectively. Also, m represents the mass of the
catalyst.
Typically, in a glass reactor, a solution of the catalyst (0.5 g
Lꢁ1, 1 mL), NaBH4 (0.4 mol Lꢁ1, 0.75 mL) and nitroarenes
(1 mmol Lꢁ1, 8 mL) were stirred. The reaction was then
quenched by adding NH4Cl solution and the reaction mixture
was extracted with ethyl acetate. Subsequently, the mixture was
dried using MgSO4. The product obtained was analyzed using
gas chromatography (GC-2010 Plus, capillary column, KB-1) and
gas chromatography-mass spectrometry (GC-MS, Thermo-
Finnigan Trace DSQ coupled with a HP-5MS column).
3. Results and discussion
3.1 Characterization of the as-synthesized catalysts
Fig. 2 SEM images of as-prepared NiMoOx/MC catalysts with different
morphologies. Note: (a) hexagonal prisms, (b) long rods, (c) platelets,
and (d) sphericals.
The XRD patterns of different morphologies of the catalysts are
displayed in Fig. S1† and 1. Fig. S1(a)† and 1(a) show the small-
angle XRD patterns of NiMoOx/SBA-15 and the corresponding
NiMoOx/MC. All samples displayed a sharp diffraction peak
below 1.1ꢂ and two other peaks between 1.1ꢂ and 2.0ꢂ that can
be indexed as (100), (110) and (200) diffraction signals associ-
ated with an ordered hexagonal mesostructure. The unchanged
ordered structure was a good indication that the NiMoOx/MC
material could be replicated from NiMoOx/SBA-15. In addition,
compared with the XRD patterns of NiMoOx/SBA-15, the char-
acteristic diffraction peaks attributed to the (100) in the XRD
patterns of NiMoOx/MC shied to the wide-angle direction,
thus indicating that the pore size of NiMoOx/MC was smaller
than that of NiMoOx/SBA-15.29 Fig. S1(b)† reveals the wide-angle
XRD patterns of NiMoOx/SBA-15 catalysts. There was one broad
peak between 20ꢂand 30ꢂfor all the catalysts, which could be
indexed to amorphous silica. One peak centered at 27.5ꢂ was
attributed to the (011) crystal plane of MoO3 (JCPDS card 47-
1320).30,31 Interestingly, the dispersion of MoO3 was different
with respect to the support morphology changes. Among the
four samples, the (011) peak intensity of MoO3 in NiMoOx/SBA-
15-PL was higher than those of the other catalysts, which indi-
cated that MoO3 had poor dispersibility in NiMoOx/SBA-15-PL.
The wide-angle XRD patterns for NiMoOx/MC with different
morphologies exhibited three main diffraction peaks, as illus-
trated in Fig. 1(b). One wide diffraction peak between 20ꢂ and
30ꢂ was attributed to carbon. The diffraction pattern at 14.3ꢂ
originated from the NiMoO4 phase (JCPDS card 33-0948), which
was ascribed to (110). Another diffraction pattern at 44.5ꢂ
originated from metallic Ni (JCPDS card 04-0850), which was
assigned to (111) of metal Ni.32,33
respectively. Moreover, the length and width of NiMoOx/MC-HP
were ꢀ755 nm and ꢀ530 nm, respectively. For NiMoOx/MC-SP,
the particle diameter was measured to be ꢀ745 nm. From Fig. 2,
it can be seen that these four catalysts retained the original
The surface morphology of the NiMoOx/MC catalysts was
investigated by SEM, as shown in Fig. 2. The length and width of
NiMoOx/MC-PL were ꢀ1075 nm and ꢀ350 nm, and those of the
Fig. 3 TEM images and NiMoOx particles size distribution histograms
of NiMoOx/MC. Note: (a–c) NiMoOx/MC-HP, (d–f) NiMoOx/MC-PL,
NiMoOx/MC-LR catalyst were ꢀ820 nm and ꢀ230 nm, (g–i) NiMoOx/MC-LR and (j–l) NiMoOx/MC-SP.
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Fig. 4 HRTEM images and the corresponding EDS elemental mapping images of (a–d) NiMoOx/MC-PL, (e–h) NiMoOx/MC-PL-450 and (i–l)
NiMoS/MC-PL.
morphology of their corresponding NiMoOx/SBA-15, indicating the NiMoOx/MC-PL and NiMoOx/MC-PL-450 catalysts are shown
that the high calcination temperature did not destroy the in Fig. 4, which reveal that the three elements were homoge-
structures of NiMoOx/SBA-15 during the replication process.
neously distributed in the structure of the catalyst.34 The molar
The TEM images and NiMoOx NPs size distribution histo- ratio of Ni to Mo was about 2, which was obtained from the
grams of NiMoOx/MC are shown in Fig. 3. From Fig. 3(a, d, g TEM-EDS of NiMoS/MC-PL (Fig. S2†).
and j), it can be seen that all catalysts had a highly-ordered
The nitrogen adsorption–desorption isotherms, pore size
mesoporous structure, which was consistent with the small- distribution, and the corresponding parameters of the catalysts
angle XRD patterns. Importantly, ultrane NPs were highly with different morphologies are displayed in Fig. 5 and Table 1.
dispersed without any aggregation and well-encapsulated into The nitrogen adsorption–desorption isotherms exhibited
the mesopores. As seen in the size distribution histograms, the typical type IV patterns with hysteresis loops and capillary
average sizes of the NiMoOx NPs of NiMoOx/MC-HP, NiMoOx/ condensation, which indicated the presence of mesopores.35,36
MC-LR, NiMoOx/MC-PL, and NiMoOx/MC-SP were ꢀ0.97, The pore size distributions of NiMoOx/MC-PL and NiMoOx/MC-
ꢀ1.10, ꢀ1.37 and ꢀ1.65 nm, respectively. Furthermore, the SP were about 4.0 nm, which indicated that the as-synthesized
corresponding elemental mapping images of Ni, Mo, and O for mesoporous materials had a uniform pore size distribution.37
Fig. 5 The N2 adsorption–desorption isotherms and pore size distribution of (a) NiMoOx/MC-PL, (b) NiMoOx/MC-SP, (c) NiMoOx/MC-HP, and
(d) NiMoOx/MC-LR.
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Table 1 The structural characteristics of mesoporous NiMoOx/MC
SBETa (m2 gꢁ1
)
Smicb (m2 gꢁ1
)
Vmic (cm3 gꢁ1
)
Vmes (cm3 gꢁ1
)
dBJHd (nm)
c
Samples
Dimensions (ca. nm)
NiMoOx/MC-PL
NiMoOx/MC-LR
NiMoOx/MC-SP
NiMoOx/MC-HP
1075 ꢃ 350
820 ꢃ 230
755 ꢃ 530
745 ꢃ 745
995
795
762
673
42
360
185
298
0.02
0.19
0.09
0.15
0.98
0.81
0.66
0.61
4.0
5.0
3.9
4.5
a
b
c
d
BET surface area. Calculated using the t-plot micropore area. Calculated using the t-plot micropore volume. Pore diameter was calculated
with the BJH method.
The H2-TPR technique was used to study the reducibility of
the Ni and Mo oxides on the support. The proles are listed in
Fig. 6. The proles of the as-prepared catalysts exhibited two
ꢂ
ꢂ
wide peaks in the 320–430 C and 590–640 C ranges.
The rst reduction peak was derived from nickel oxide and
a portion of the molybdenum oxide. The second peak was
ascribed to the second step reduction of MoOx. The different
peak reduction temperatures were ascribed to the differences in
the reducibility of the oxide. Compared to the Mo species, the Ni
species was easier to reduce. According to the reference, the
reduction peak for pure NiO (Ni2+ to Ni0) was centered around
350–400 C38 and the reduction peak for MoO3 (Mo6+ to Mo4+
)
ꢂ
appeared at about 430 C.39 Compared to pure NiO, the reduc-
ꢂ
Fig. 6 H2-TPR profiles of catalysts (a) NiMoOx/MC-PL, (b) NiMoOx/
MC-LR, (c) NiMoOx/MC-HP and (d) NiMoOx/MC-SP.
tion peak of the different catalysts for the Ni species migrated to
a
lower temperature. This result was attributed to the
phenomenon of hydrogen overow from molybdenum oxide to
nickel, which promoted the reduction of nickel oxide.40
As shown in Table 1, the surface area of NiMoOx/MC-PL was
ꢀ995 m2 gꢁ1, which was larger than those of NiMoOx/MC-HP,
NiMoOx/MC-LR, and NiMoOx/MC-SP. Therefore, the NiMoOx/
MC-PL catalyst provided a large surface area for the dispersion
of the active sites. The average pore size of the NiMoOx/MC
catalyst followed the order: NiMoOx/MC-LR (5.0 nm) > NiMoOx/
MC-HP (4.5 nm) > NiMoOx/MC-PL (4.0 nm) > NiMoOx/MC-SP
(3.9 nm).
XPS is a surface analytical technique that reveals information
on the chemical state and surface composition of the catalysts.
The XPS spectra and related parameters for NiMoOx/MC-PL,
NiMoOx/MC-PL-450, NiMoOx/MC-PL-750, and NiMoS/MC-PL
are displayed in Fig. 7 and Table S1.† The Ni 2p spectra
showed four main peaks, which could be assigned to Ni0, NiO,
Ni(OH)2, and the satellite signal.41,42 The ratio dened as Ni0/
Fig. 7 XPS spectra of Ni 2p and Mo 3d for different catalysts. Note: (a) NiMoOx/MC-PL, (b) NiMoOx/MC-PL-450, (c) NiMoOx/MC-PL-750, and (d)
NiMoS/MC-PL.
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Fig. 8 The reduction of 4-NP recorded every 1.5 min over NiMoOx/MC-PL (a), NiMoOx/MC-HP (b), NiMoOx/MC-SP (c), and NiMoOx/MC-LR (d).
The relationship between conversion and reaction time (e). Plots of ln(Ct/C0) versus reaction time (f).
(Ni0+Ni2+) followed the order of NiMoOx/MC-PL-750 > NiMoOx/
3.2 Catalytic activity towards the reduction of 4-nitrophenol
MC-PL-450 > NiMoOx/MC-PL, which correlated well with the H2-
The catalytic performance of the catalysts was evaluated by the
TPR spectra. The results showed that the content of metallic
reduction of 4-nitrophenol (4-NP) in the presence of NaBH4. The
nickel and the active site increased with the increase in the
reduction process of 4-NP was monitored by measuring the UV-
reduction temperature. The Mo 3d XPS spectra for the different
Vis absorbance of the solution. According to the reference, the
aqueous solution of 4-NP showed a strong absorption band at
317 nm. The absorption band shied to 400 nm, when NaBH4
was added into the above solution, which indicated the
formation of the nitrophenolate ion.43 The catalytic properties
of NiMoOx/MC with different morphologies towards the
reduction of 4-NP were comparatively studied, as displayed in
Fig. 8. With the prolongation of the reaction time, the intensity
of absorption peak at 400 nm decreased while another
absorption band at 300 nm appeared.44 Aer the completion of
catalysts showed several pairs of corresponding peaks, which
could be assigned to Mo6+, Mo4+, and metal Mo0. Compared to
NiMoOx/MC-PL, the binding energy of Mo4+ and Mo6+ in
NiMoOx/MC-PL-450 and NiMoOx/MC-PL-750 shied to lower
values, respectively, indicating that more unsaturation sites
were generated. From Table S1,† the percentage of MoOx
species, dened as (Mo6+ + Mo4+)/(Mo0 + Mo4+ + Mo6+), exhibi-
ted a signicant change in different catalysts; it dropped from
an initial of 88% to 63.7% with the increase in the reduction
temperature. These results indicated that the molybdenum
the reaction, the peak corresponding to the nitrophenolate ion
oxide on the surface of NiMoOx/MC was not completely
was not observed, revealing that 4-NP was completely reduced.
reduced, even when the temperature was relatively high. In the
From Fig. 8(e), it can be observed that the reaction times for the
case of NiMoS/MC-PL, the XPS peaks of Ni and Mo demon-
conversion of 50% of 4-NP to 4-AP in the presence of NiMoOx/
strated the formation of the NiMoS nanocrystal.
MC-PL, NiMoOx/MC-HP, NiMoOx/MC-SP, and NiMoOx/MC-LR
Table 2 The kinetic constants, turnover frequencies (TOF), and metal Ni loading with different catalysts
Samples
k (minꢁ1
)
K (minꢁ1 gꢁ1
)
Ni loadinga (wt%)
TOFb ꢃ 103 (sꢁ1
)
NiMoOx/MC-PL
NiMoOx/MC-HP
NiMoOx/MC-SP
NiMoOx/MC-LR
NiMoOx/MC-PL-450
NiMoOx/MC-PL-750
NiMoS/MC-PL
NiO/MC-PL
0.48
0.12
0.10
0.09
0.65
0.18
0.03
0.16
0.002
0.17
0.001
960
240
200
180
1300
360
60
320
4
2073
2
4.05
3.2
175.8
69.3
64.7
49.9
241.5
53.8
—
56.2
—
103.8
—
2.78
3.37
4.05
4.05
4.05
4.05
—
MoO3/MC-PL
NiMoOx
MC-PL
24.7
—
a
Determined by ICP-MS. b Calculated on the basis of total catalyst and a conversion rate of 50%.
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Fig. 9 Catalytic reduction of 4-NP for a time period of 1.5 min over NiMoOx/MC-PL (a), NiO/MC-PL (b), MoO3/MC-PL (c), NiMoOx (d), and MC-
PL (e). The conversion of 4-NP with reaction time over different catalysts (f). The relationship between At/A0 and time (g). Plots of ln(Ct/C0) versus
time (h).
catalysts were 2.0 min, 6.6 min, 8.2 min, and 8.7 min, respec- NiMoOx/MC-PL,
NiMoOx/MC-HP,
NiMoOx/MC-LR,
and
tively. To qualify the reactivity, the reaction was regarded as NiMoOx/MC-SP was mainly attributed to their different pore
a pseudo-rst-order reaction. From Fig. 8(f), it can be observed diameters, channel lengths, and surface areas. The NiMoOx/
that the reaction time is linear with respect to ln(Ct/C0), which MC-PL catalyst had short channels, which shortened the
was consistent with rst order kinetics. In order to compare the diffusion route for the reactants in the pores. This made the
catalytic performances of different catalysts, the rate constants, reactants to have easier contact with the active sites, thus
k (minꢁ1) and K (minꢁ1 gꢁ1), and the value of TOF (sꢁ1) of the favoring 4-NP reduction. In addition, the NiMoOx/MC-PL cata-
catalysts were calculated. It can be found (Table 2) that the lyst exhibited a super catalytic performance owing to its large
morphology of the catalysts had an important effect on the K specic surface area (ꢀ995 m2 gꢁ1) and short channel length
and TOF values. Among the four different morphologies of the (ꢀ351 nm). A larger specic surface area provided more
catalysts, the NiMoOx/MC-PL catalyst exhibited the highest K attachment sites for NiMoOx NP and then increased the active
(960 minꢁ1 gꢁ1) and TOF (0.176 sꢁ1) values. According to the sites. However, NiMoOx/MC-HP, NiMoOx/MC-LR, and NiMoOx/
HRTEM results, the size of the NiMoOx NPs of the catalysts was MC-SP catalysts had long channel lengths, which made 4-NP to
similar. Therefore, the difference in the catalytic activities of
Fig. 10 Catalytic reduction of 4-NP over NiMoOx/MC-PL (a), NiMoOx/MC-PL-450 (b), NiMoOx/MC-PL-750 (c), and NiMoS/MC-PL (d). Plots of
the conversion of 4-NP versus time (e). The relationship of ln(Ct/C0) and the reaction time (f).
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Table 3 The reduction of nitroarenes over the NiMoOx/MC-PL-450 catalysta
Entry
1
Substrate
Product
Time (min)
10.0
Conv. (%)
97.2
k (minꢁ1
)
K (minꢁ1 gꢁ1
)
TOFb ꢃ 103 (sꢁ1
)
—
—
37.6
2
3
4
40.0
10.0
20.0
68.1
97.5
88.4
0.039
—
78
6.6
37.7
17.1
—
0.128
256
5
6
10.0
40
99
99
—
—
38.3
9.6
0.039
78
7
40
61.5
66.5
0.018
0.287
36
6.0
8
10.0
574
25.7
a
Reaction conditions: catalyst (0.5 g Lꢁ1, 1 mL), nitroarenes (1 mmol Lꢁ1, 8 mL), NaBH4 (0.4 mol Lꢁ1, 0.75 mL), 600 rpm, room temperature. b The
turnover frequencies (TOF) denote the moles of 4-NP converted per mole of Ni or Mo per second, calculated on the basis of total Ni and Mo loading
in this research system.
diffuse for a longer period of time in contact with the active sites catalyst had the largest TOF, indicating that the Ni species in
and therefore exhibited relatively inferior catalytic activities.
the NiMoOx/MC was the main active site, while the Mo species
To further investigate the effect of composition on the contributed to the performance of the Ni catalyst. Compared
reduction of 4-NP, the NiO/MC-PL, MoO3/MC-PL, NiMoOx and with NiMoOx/MC-PL, the NiMoOx catalyst exhibited almost no
MC-PL catalysts were also prepared and compared with catalytic activity. The enhanced catalytic properties of the
NiMoOx/MC-PL. For NiMoOx/MC-PL, NiO/MC-PL, and NiMoOx, NiMoOx/MC-PL catalyst could be attributed to the synergistic
the reaction times for the conversion of 50% of 4-NP were 2.0, effect of Mo and Ni species, as well as the strong interaction
6.5 and 3.5 min in (Fig. 9), respectively. However, 4-NP displayed between the metal species and MC.
almost no change over the MoO3/MC-PL and MC-PL catalysts
To further study the inuence of valence-states of Ni and/or
with the reaction time of 14.0 min, indicating that the sole Mo Mo on the catalytic performance, the NiMoOx/MC-PL catalyst
species and MC exhibited no activity for the 4-NP reduction. As was reduced by hydrogen at different temperatures.
shown in Table 2, the composition of the catalyst played an
NiMoS/MC-PL was also prepared for comparison. It can be
important effect on the values of K and TOF. The TOF values for concluded from Fig. 10(e) that the conversion of 4-NP followed
these catalysts followed the order of NiMoOx/MC-PL (0.176 sꢁ1
)
the order of NiMoOx/MC-PL-450 > NiMoOx/MC-PL > NiMoOx/
> NiMoOx (0.104 sꢁ1) > NiO/MC-PL (0.056 sꢁ1) > MoO3/MC-PL (0 MC-PL-750 > NiMoS/MC-PL. From Table 2, it can be found that
sꢁ1) ¼ MC-PL (0 sꢁ1). It was evident that the NiMoOx/MC-PL the valence-states of the metal also played a great role in the K
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and TOF. The NiMoOx/MC-PL-450 catalyst had the largest K
(1300 minꢁ1 gꢁ1) and TOF (0.242 sꢁ1) values. Among these four
catalysts, NiMoOx/MC-PL-450 exhibited the highest TOF value,
which was attributed to the excellent synergistic effect between
Ni and Mo species. According to the results of H2-TPR and XPS,
aer reduction at temperature 450 ꢂC, most of the Ni species is
converted to metallic nickel, while the MoOx species is still
present. The results demonstrate that the metallic Ni had more
activity than NiO towards the reduction of 4-NP. Moreover,
metallic Ni and MoOx possessed the best synergistic effect for
the reduction of 4-NP. With regard to the catalyst reduced at
750 ꢂC, the proportion of Mo0 and Mo4+ species increased.
However, the activity of NiMoOx/MC-PL-750 was lower than that
of NiMoOx/MC-PL-450, clarifying that MoO3 could contribute
more to the reduction properties of Ni compared to metallic Mo
and MoO2. In addition, NiMoS/MC-PL showed a poor reaction
rate, which indicated that the NiMoS species was less active
than the corresponding metallic and oxidized NiMoOx species.
3.4 The universal applicability of the NiMoOx/MC-PL-450
catalyst towards the reduction of nitroarene
In order to demonstrate the universal applicability of the
NiMoOx catalyst in reducing the nitroarenes to aniline, various
nitroarenes with different substituents were selected as the
substrates.45 NiMoOx/MC-PL-450 was used as the catalyst and
the results are listed in Table 3. NiMoOx/MC-PL-450 exhibited
an efficient catalytic activity to selectively reduce nitroarenes
with different functionalities. Almost no by-products were
detected. The nitroarenes were converted to the corresponding
amines in the yield of >84.0% (Table 3, entries 1, 3, 4, 5 and 6),
while 4-nitroanisole and 2-amino-4-nitrotoluene showed a rela-
tively low conversion (less than 70%) under the same condi-
tions. The NiMoOx/MC-PL-450 catalyst demonstrated a high
performance towards the reduction of nitroarenes under the
optimized conditions. Therefore, the NiMoOx/MC-PL-450 cata-
lyst is of great signicance for the reduction of nitroarenes with
different functional groups.
3.3 The effect of reaction parameters on the reduction of 4-
NP
3.5 The proposed mechanism towards the reduction of
nitroarenes
It is well known that the reaction rate of a chemical reaction is
related to the concentration of reactants and other factors. In
this research, the amount of catalyst and 4-NP as well as the
concentration of NaBH4 were systematically studied. The
concentration of NaBH4 was varied, while all other parameters
were kept constant (the concentration of 4-NP was 12 mM and
the amount of catalyst was 0.5 mg). From Fig. S3 and Table S2,†
it can be seen that the reaction rate increased as the concen-
tration of NaBH4 increased. Interestingly, the reaction rate
increased rapidly as the concentration of NaBH4 ranged from
0.005 to 0.02 M, whereas the rate increased slowly as the
concentration of NaBH4 ranged from 0.02 to 0.04 M. As we can
see from the Table S2,† the variation tendency of TOF was
similar to that of the rate constant as the concentration of
NaBH4 changed. It is concluded that the reduction rate and TOF
were closely related to the concentration of NaBH4.
According to the above experimental results, the NiMoOx/MC-
PL-450 catalyst exhibited a high performance, which was
mainly ascribed to the synergistic effect between metal Ni and
MoOx species. A possible mechanism was proposed towards the
reduction of nitroarenes in the presence of NaBH4 over the
NiMoOx/MC-PL-450 catalyst in Scheme 2.46 The typical steps are
described below. First, the hydrolysis of NaBH4 produced active
hydrogen and electrons that were adsorbed on the surface of the
NiMoOx catalyst. Then, the active hydrogen was transferred
from the surface of the NiMoOx catalyst to nitroarenes, which
were adsorbed on the catalyst surface. Second, the nitroso
intermediates were formed aer the loss of water molecules.
The intermediates were then rapidly reduced to a stable
hydroxylamine. Finally, the nal product amine was obtained
The effect of the amount of catalyst on the reduction of 4-NP
is shown in Fig. S4 and Table S3.† It can be observed that k
increased as the amount of catalyst increased. As seen in
Fig. S4(h),† k was almost linear with respect to the amount of
catalyst in the reaction system. This indicated that the increase
in the catalyst amount resulted in more surface active sites.
More chemical reactions then occurred, thus increasing the
overall rate. The effect of the dosages of 4-NP on this reaction is
shown in Fig. S5 and Table S4.† From Fig. S5(h),† it can be
found that the reaction rate had a indirect relationship with the
amount of 4-NP. The reaction rate decreased as the amount of 4-
NP increased. This phenomenon can be ascribed to the
Fig. S5(h),† it can be found that the reaction rate decreases as
the amount of 4-NP increases. This phenomenon can be
ascribed to the competition between the reactants and free sites
on the surface of NiMoOx NPs. When reactants were adsorbed
on the surface of the catalyst, the reaction occurred. In addition,
adsorption of excessive reactants reduced the whole reaction
rate.
Scheme 2 The mechanism towards the reduction of nitroarenes over
the NiMoOx/MC-PL-450 catalyst.
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There are no conicts to declare.
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Acknowledgements
This study was supported by the National Natural Science
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