A robust core-shell nanostructured nickel-iron alloy@nitrogen-containing carbon catalyst for the highly efficient hydrogenation of nitroarenes

Article abstract of DOI:10.1039/c8dt03033b

Currently, the catalytic selective hydrogenation of nitroarenes to produce aromatic amines is one of the most important key reactions in many fine chemical processes. In particular, non-noble-metal-catalyzed hydrogenation of nitroarenes represents more sustainable chemical processes. Here, we report a new robust and recyclable core-shell nanostructured nickel-iron alloy@nitrogen-containing carbon (NiFe@NC) catalyst and the beneficial effect of alloying Ni with Fe for the above reaction. The key to this synthetic strategy was thermally transforming the Ni-Fe layered double hydroxide (NiFe-LDH)/melamine mixture to form a fixed NiFe@NC nanostructure. A series of characterization results revealed the formation of NiFe alloy nanoparticles (NPs) coated with the NC overlayer. The as-fabricated NiFe@NC catalyst with a Ni/Fe atomic ratio of 3.0 exhibited superior activity for the reduction of the nitro group in o-chloronitrobenzene, with a 99.5% yield of o-chloroaniline under mild reaction conditions. The initial reaction rate over the catalyst was nearly three times that over the monometallic Ni@NC counterpart, and even one-order magnitude higher than that over pristine NiFe-LDH-derived NiFe alloy NPs. The extraordinary activity of NiFe@NC was reasonably attributed to the unique core-shell nanostructure, where both the NiFe alloy core and the NC overlayer shell could construct a significant promotional effect, being beneficial for the selective cleavage of the N-O bond. Recycling experiments indicated that the catalyst could be easily separated and recovered under an external magnetic field and experienced excellent recyclability during seventeen cycles without an obvious loss of catalytic activity. Furthermore, the present catalyst was also highly active for the chemoselective hydrogenation of other substituted nitroarenes bearing different functional groups to the corresponding anilines.

Full text of DOI:10.1039/c8dt03033b

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A robust core–shell nanostructured nickel–iron
alloy@nitrogen-containing carbon catalyst for the
highly efficient hydrogenation of nitroarenes†
Cite this: DOI: 10.1039/c8dt03033b
Yaowen Zhang, Chunling Liu, Guoli Fan, Lan Yang
and Feng Li
*
Currently, the catalytic selective hydrogenation of nitroarenes to produce aromatic amines is one of the
most important key reactions in many fine chemical processes. In particular, non-noble-metal-catalyzed
hydrogenation of nitroarenes represents more sustainable chemical processes. Here, we report a new
robust and recyclable core–shell nanostructured nickel–iron alloy@nitrogen-containing carbon
(NiFe@NC) catalyst and the beneficial effect of alloying Ni with Fe for the above reaction. The key to this
synthetic strategy was thermally transforming the Ni–Fe layered double hydroxide (NiFe-LDH)/melamine
mixture to form a fixed NiFe@NC nanostructure. A series of characterization results revealed the for-
mation of NiFe alloy nanoparticles (NPs) coated with the NC overlayer. The as-fabricated NiFe@NC cata-
lyst with a Ni/Fe atomic ratio of 3.0 exhibited superior activity for the reduction of the nitro group in
o-chloronitrobenzene, with a 99.5% yield of o-chloroaniline under mild reaction conditions. The initial
reaction rate over the catalyst was nearly three times that over the monometallic Ni@NC counterpart, and
even one-order magnitude higher than that over pristine NiFe-LDH-derived NiFe alloy NPs. The extraordi-
nary activity of NiFe@NC was reasonably attributed to the unique core–shell nanostructure, where both
the NiFe alloy core and the NC overlayer shell could construct a significant promotional effect, being ben-
eficial for the selective cleavage of the N–O bond. Recycling experiments indicated that the catalyst
could be easily separated and recovered under an external magnetic field and experienced excellent
recyclability during seventeen cycles without an obvious loss of catalytic activity. Furthermore, the
present catalyst was also highly active for the chemoselective hydrogenation of other substituted nitro-
arenes bearing different functional groups to the corresponding anilines.
Received 25th July 2018,
Accepted 5th September 2018
DOI: 10.1039/c8dt03033b
rsc.li/dalton
2
–4
are available for the reduction of nitroarenes.
selective reduction of the nitro group in chloronitrobenzenes
Over the past half century, hydrogenation reactions have (CNBs), however, the undesired hydrodehalogenation side
As for the
1
. Introduction
5
,6
gained considerable attention due to their importance in both reaction is often involved, besides the formation of main
the academic field and the industrial production of fuels and chloroanilines (CANs), as well as hydroxylamine, hydrazine,
chemicals. In particular, metal-catalyzed selective hydrogen- azoarene, or azoxyarene intermediates, due to the poor activi-
ation of nitroarenes to produce subsequent aromatic amines ties of catalysts. Despite high conversions of CNBs in this type
has been extensively investigated, because aromatic amines of reaction conducted over a variety of noble metal catalysts
7
,8
9
10,11
12,13
14
can be widely used as an important feedstock for the commer- (e.g. Pd, Ru, Pt,
Au
and Ag ), the high cost of noble
cial manufacturing of pharmaceuticals, dyes, herbicides, pesti- metals greatly limits their practical applications. Recently,
1
cides, and cosmetic products. Now, the worldwide demand many efforts have been made towards using low-cost first-row
1
5–17
for aromatic amines is substantially increasing. Various pro- transition metal catalysts, such as Ni/TiO
2
and Fe
2
O
3
,
as
cedures involving heterogeneous catalysts and reducing agents well as heteroatom-doped carbon spheres as metal-free cata-
1
8
lysts. In this regard, nickel-based catalysts show relatively
1
9–21
high selectivity to aromatic amines,
which represents
State Key Laboratory of Chemical Resources Engineering, Beijing Advanced
Innovation Center for Soft Matter Science and Engineering, Beijing University of
Chemical Technology, Beijing, 100029, China. E-mail: lifeng@mail.buct.edu.cn;
Fax: +86 10 64425385; Tel: +86 10 64451226
more sustainable chemical processes. Their activities, however,
usually are much lower than those of noble metal-based ones.
Moreover, in some cases, weak metal–support interactions can
reduce the stability of catalysts. As a result, developing cost-
effective non-noble-metal catalysts with robust catalytic activity
†
Electronic supplementary information (ESI) available: Additional experimental
results. See DOI: 10.1039/C8DT03033B
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and stability is a key issue when it comes to reducing nitro-
arenes to produce aromatic amines.
2
. Experimental
2.1 Preparation of samples
As one of the effective strategies for improving catalytic per-
formance, elaborately designing or manipulating surface NiFe-LDH precursors with different Ni/Fe molar ratios of x (x =
microstructures of catalysts always benefits the adsorption of 2.0, 3.0, 4.0) were prepared by our developed separate nuclea-
34
reactants and thus the activation. Therefore, much attention tion and aging steps method. In a typical synthesis, 100 mL
has been paid to exploring alternative support materials to of salt solution containing Ni(NO ·6H and Fe
a
3
)
2
2
O
strengthen metal–support interactions and thus improve the (NO ) ·9H O and 100 mL of a base solution containing
3
3
2
2
−
3+
−
2+
reusability and recyclability of catalysts. Recently, as a kind of Na
2
CO
3
and NaOH ([CO
3
] = 2[Fe ], [OH ] = 1.6([Ni ] +
3
+
fascinating carbon material, nitrogen-containing carbon (NC) [Fe ])) were simultaneously added into a colloid mill and vig-
2
2,23
has been gaining considerable interest in the whole world,
orously mixed at a rotor speed of 4000 rpm for 3 min.
because the introduction of highly electron-rich nitrogen Subsequently, the resulting suspension was aged at 60 °C for
atoms into the carbon framework can greatly improve the elec- 6 h and centrifuged. The obtained precipitate was washed with
trical and chemical properties of carbon materials, thereby deionized water until the pH value reached about 7.0. The syn-
2
4,25
bringing out a wide range of practical applications.
In par- thesized NiFe-LDH was dried at 70 °C overnight in a vacuum
ticular, loading metallic nanoparticles (NPs) on NC materials oven, and then mixed with melamine (the mass ratio of mela-
may markedly affect the electronic character of metal mine to NiFe-LDH is set at 3.0) to obtain a mixture (denoted as
2
6,27
species.
Moreover, the metallic or semimetallic nature of NiFe-LDH/M). Finally, NiFe-LDH/M was calcined under a N
2
nanocarbons ensures strong electronic interactions with metal atmosphere at 600 °C for 2 h at a ramp rate of 5 °C to obtain
species to a large extent.
different NiFe@NC-x samples.
On the other hand, layered double hydroxides (LDHs),
For comparison, pure NiFe-LDH with a Ni/Fe molar ratio of
known as a class of two-dimensional highly ordered layered 3.0 was reduced under a 5% H
2
/Ar atmosphere at 600 °C in the
2
+
3+
materials, are mainly composed of metal M and M cations absence of melamine to obtain the NiFe-3 sample. In addition,
2
8,29
distributed uniformly in the brucite-like layers.
3 2 2
Many the mixture of Ni(NO ) ·6H O and melamine (the melamine/
advantages including low price, ease of preparation, and struc- Ni(NO
3
)
2
·6H O mass ratio = 3.0) was calcined according to the
2
tural variability provide LDH materials high potential for same procedure as that for NiFe-LDH/M samples. The
various practical applications. In particular, numerous obtained sample was denoted as Ni@NC. Meanwhile, NiFe-
research efforts have successfully integrated LDHs into the syn- free pure NC was prepared from melamine without the
thesis of a variety of highly dispersed supported metal or addition of NiFe-LDH under identical experimental
metal oxide catalysts through the structural transformation of conditions.
3
0–32
LDH precursors.
Recently, we have reported that LDH-
derived Ni-based catalysts possess good catalytic activity in the
2
0,33
2.2 Sample characterization
conditions employed (e.g. high reaction temperature and high X-ray diffraction (XRD) patterns of all samples were recorded
hydrogen partial pressure). on a Shimadzu XRD-6000 diffractometer using a Cu Kα radi-
As we know, metal catalysts often suffer from several draw- ation source (λ = 0.15418 nm).
o-CNB hydrogenation to o-CAN,
despite the harsh reaction
backs, such as aggregation, separation and leaching of active
Elemental analysis of the metal contents was performed on
metal components. So, constructing magnetic metal NPs a Shimadzu ICPS-7500 inductively coupled plasma atomic
embedded into carbon nanomaterials will create a new emission spectroscope (ICP-AES). Before measurement, the
opening for metal catalysts because of their ease of magnetic sample was first calcined at 700 °C under air for 2 h, and then
separation, cost effectiveness and high stability. Herein, we the remainder was dissolved in aqua regia. The carbon and
report a novel low-cost and magnetically recyclable core–shell nitrogen contents were determined by elemental microanalysis
structured Ni–Fe alloy/NC (NiFe@NC) nanocatalyst fabricated (Elementar Vario analyzer).
by the thermal decomposition of the NiFe-LDH/melamine
The morphology and microstructures of the samples were
mixture. The as-fabricated NiFe@NC catalyst shows excellent observed using a field emission scanning electron microscope
catalytic performance in the liquid phase selective hydrogen- (FE-SEM, Zeiss Supra 55); transmission electron microscopy
ation of o-CNB to o-CAN in terms of activity, selectivity and (TEM) and high-resolution TEM (HRTEM) were conducted
stability. By virtue of the core–shell nanostructure composed using a JEOL JEM-3010 electron microscope. High-angle
of a NiFe alloy core and a thin NC layer shell, strong metal– annular dark-field scanning TEM (HAADF-STEM) images were
support interactions (SMSIs) can occur. Detailed characteriz- observed on a JEOL2010F instrument combined with an
ation reveals that the beneficial effect of alloying of Ni with Fe energy dispersive X-ray spectrometer (EDX).
is the result of the electronic modification of Ni and thus
Low temperature nitrogen adsorption–desorption isotherms
enhances the catalytic performance of the bimetallic catalyst. of the samples were obtained using a Micromeritics ASAP 2020
What’s more, the unique core–shell nanostructure effectively sorptometer apparatus. Specific surface areas were measured
prevents the agglomeration of NiFe alloy NPs, thus remarkably by the multipoint BET method, while total pore volumes and
improving the catalyst stability.
average pore sizes were calculated by the BJH method.
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X-ray photoelectron spectroscopy (XPS) measurements were ities were obtained based on at least 3 parallel experiments
conducted on a Thermo VG ESCALAB250 X-ray photoelectron with experimental errors of less than 3%. The values of the
spectrometer using an Al Kα X-ray radiation source.
A vibrating sample magnetometer (VSM, Lake Shore 7410) of o-CNB converted per mole of surface metallic sites deter-
was used to measure the magnetic property of the samples. mined by H pulse chemisorption.
Temperature-programmed desorption of CO (CO -TPD)
experiments were conducted using a Micromeritics AutoChem
920 II chemisorption apparatus with a thermal conductivity
turnover frequency (TOF) were calculated based on the moles
2
2
2
2
3
. Results and discussion
detector (TCD). The sample (100 mg) was degassed at 300 °C
3.1 Sample characterization
for 1 h under a He flow. Then a CO
2
stream (40 mL min− ) was
1
As shown in Fig. 1A, the XRD patterns of pristine NiFe-LDH
present a series of characteristic diffractions for the (003),
switched into the sample tube at 100 °C for 1.5 h.
Subsequently, a He flow was switched at 100 °C for 2 h.
(
006), (012), (015), (110) and (113) planes related to the carbon-
Finally, the temperature was increased to 800 °C at a rate of
1
Oxygen temperature-programmed oxidation (O -TPO)
2
experiments were performed on a Thermo Fisher TPDRO-1100
chemical adsorption instrument with a TCD. The sample
0 °C min− under a He gas flow.
1
ate-type LDH phase (JCPDFS no. 38–0487). For NiFe-LDH/M
mixtures, intensive characteristic diffractions for the melamine
phase (JCPDS no. 41-1390) can be detected. Fig. 1B shows the
XRD patterns of reduced samples. Conspicuously, three inten-
sive characteristic diffractions for the (111), (200) and (220)
(30 mg) was degassed at 350 °C for 1 h under a He flow. TPO
planes of the face centered cubic (fcc) Ni Fe alloy phase
was performed at 30 °C in a stream of 10% v/v O /He at a
2
3
−
1
(
JCPDFS no. 38-0419) can be found in the cases of NiFe@NC
heating rate of 5 °C min
pulse chemisorption was conducted on a Micromeritics
AutoChem II 2920 instrument. First, the sample (100 mg) was
.
samples. Meanwhile, small amounts of metallic nickel (JCPDS
no. 04-0850) and nickel oxide (bunsenite, JCPDS no. 47-1049)
phases are detected in two cases of NiFe@NC-2 and
H
2
−
1
degassed under a flow of Ar (40 mL min ) at 500 °C for 1 h
and then the temperature was decreased to 55 °C and sub-
0
NiFe@NC-3. For NiFe@NC-4, no metallic Ni is observed, prob-
ably attributable to the highly dispersive nature of the formed
Ni NPs with poor crystallinity. In all cases, the absence of
carbon nitride or graphite carbon suggests the poor crystalline
nature of the carbon-containing component. The above results
suggest that the in situ formed carbon component or released
ammonia upon heating melamine in the NiFe-LDH/M mix-
tures under an inert atmosphere can serve as reducing agents
to reduce Ni and Fe species. The average particle sizes of
NiFe NPs in the NiFe@NC samples estimated based on the
Scherrer equation range from about 7.9 to 26.1 nm (Table 1),
which are smaller than that in the NiFe-LDH-derived NiFe-3
sample (38.6 nm). As shown in Fig. S1,† different NiFe@NC
samples show characteristic nitrogen adsorption–desorption
isotherms of type IV with a H1-type hysteresis loop, which is
characteristic of the mesoporous framework with narrow pore
size distributions. In particular, the NiFe@NC-3 sample pos-
2
jected to H chemisorption according to a pulse titration pro-
cedure. The total hydrogen uptake was used to calculate the
amount of surface active sites assuming that the ratio of H to
the adsorption site is 1 : 1.
Temperature programmed desorption of hydrogen (H -TPD)
2
experiments were carried out on a chemisorption analyzer
(Micromeritics AutoChem 2920 II sorptometer apparatus).
2
+
3+
First, the sample (0.1 g) was degassed under a flow of Ar
−
1
(
H
40 mL min ) at 500 °C for 1 h and treated under a flow of
/Ar (1 : 9, v/v; 40 mL min− ) at room temperature and held
for h. Finally, chemisorbed H2 was desorbed from
room temperature to 800 °C under a flow of Ar at a rate of
1
2
2
1
1
0 °C min−
.
2
.3 Catalytic test
In a typical run, the selective hydrogenation of o-CNB was con-
ducted in a batch autoclave reactor (100 mL), which was
loaded with o-CNB (0.5 g), ethanol (50 mL), and the catalyst
(0.02 g). Before the reaction, air was flushed out of the reactor
with nitrogen four times, and then the reactor was placed into
a heating jacket and preheated to a certain reaction tempera-
ture and charged with pure H to 0.5 MPa with stirring at a
2
speed of 900 rpm. During the reaction, the reaction tempera-
ture was kept constant during hydrogenation, while the hydro-
gen pressure remained unchanged by continuously supplying
hydrogen through the pressure regulator from the gas cylinder.
Finally, the liquid products were filtered and quantitatively
analyzed using a gas chromatograph (Agilent GC7890B)
equipped with a flame ionization detector and an HP-5 capil-
lary column. Under the present reaction conditions, the exter-
nal mass transfer limitation was determined to be negligible at
Fig. 1 XRD patterns of (A) melamine, NiFe-LDH and NiFe-LDH/M
samples, and XRD patterns of (B) Ni@NC (a), NiFe@NC-2 (b), NiFe@NC-3
an agitation speed of 900 rpm. The conversions and selectiv- (c), NiFe@NC-4 (d) and NiFe-3 (e) samples.
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Table 1 Analytical and structural data obtained for different samples
Content (wt%)
a
a
b
b
BET (m g−1)
c
2
pore (cm g−1)
d
3
e
f
g
Sample
Ni
Fe
N
C
S
V
d
av (nm)
D
111 (nm)
D (nm)
NiFe@NC-2
NiFe@NC-3
NiFe@NC-4
N@NC
23.5
35.5
47.0
36.6
72.5
12.8
12.9
12.8
0
10.4
8.3
6.4
10.0
0
52.0
42.2
31.7
50.9
0
40
58
46
55
3
0.198
0.335
0.168
0.338
0.004
19.02
18.72
18.93
18.20
21.17
20.8
17.9
26.1
15.8
38.6
23.9
20.6
26.2
18.5
40.1
NiFe-3
25.2
a
b
c
d
Determined by ICP-AES analysis. Determined by element microanalysis. Determined by using N
2
adsorption–desorption isotherms. Total
e
f
g
pore volume. Average pore diameter. Crystallite size of metallic NPs based on XRD line broadening of the (220) plane. The extracted surface
area weighted mean size of NPs calculated by TEM analysis.
sesses a relatively high surface area (58 m g−1) (Table 1), encapsulated by a few graphite layers with an interlayer
2
mainly because of the smaller particle size of NiFe NPs.
spacing of about 0.36 nm. Here, pyrolyzation and graphitiza-
The morphology and microstructure of the NiFe@NC tion of melamine contribute to the formation of the thin
samples were elucidated by SEM, TEM and HRTEM obser- carbon layer upon heating the LDH/M mixture. In the present
vations. SEM micrographs depict that all samples present a NiFe@NC system, such a carbon overlayer can inhibit the
homogeneous arrangement of aggregated NPs (Fig. S2†). TEM growth and aggregation of metallic NPs and strengthen the
micrographs of the samples (Fig. 2) reveal the nature of the metal–support interactions. On the other side, HRTEM images of
encapsulated nanostructure of the particles. For NiFe@NC-3, NiFe@NC-3 depict that some tiny cracks can be clearly observed
numerous NPs are embedded by the carbon overlayer, and the on the carbon layer surface (Fig. 3), as evidenced by the partial
average crystallite size (from the counting of more than 200 oxidation of metallic Ni and Fe species by air after the prepa-
particles) of encapsulated NPs is about 20.6 nm, well consist- ration procedure (see the following part). It suggests that reactant
ent with the above result based on XRD patterns. Noticeably, molecules easily get access to active sites during hydrogenation
single particles in NiFe@NC-3 show a clear contrast difference reactions, due to the presence of the unclosed shell of NC layers.
between heavier metallic NPs and the lighter carbon layer of
HAADF-STEM-EDX observation also gives more obvious evi-
about 3–8 nm thickness. Furthermore, as shown in Fig. 2e and dence of NiFe alloy NPs encapsulated into the carbon layer. As
f, two well-resolved crystalline lattice spacings of about shown in the HAADF-STEM image (Fig. 4a and c), highly dis-
0
.177 nm and 0.204 nm in well-defined cores are indexed to persed NPs in NiFe@NC-3 exist in the form of bright spots.
the (200) and (111) planes of the fcc Ni Fe alloy throughout EDX elemental mappings (Fig. 4b) reveal surface spatial distri-
3
the particles. Furthermore, it is evident that the particles are butions of Ni, Fe, C and N elements, indicating the complete
Fig. 2 TEM and HRTEM images of NiFe@NC-2 (a), NiFe@NC-3 (b, d, e, f) and NiFe@NC-4 (c). The insets in (a–c) show the particle size distributions.
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In general, thanks to the unique structure of the NiFe-LDH
precursor, Ni and Fe elements orderly distributed in the
brucite-like layers can be easily reduced to metal particles with
the help of melamine. Meanwhile, the pyrolyzation of mela-
mine following the graphitization of carbon leads to the for-
mation of the NC overlayer covering the surface of NiFe NPs,
which effectively suppresses the aggregation of NiFe NPs.
3.2 Surface properties of NiFe@NC samples
To determine the surface chemical states of C, N, Ni and Fe
species, the NiFe@NC-3 sample was characterized by XPS. XPS
survey shows the presence of Ni, Fe, C and N elements
(
(
Fig. 5a). There are three contributions in the C 1s spectrum
Fig. 5b), which are assigned to three kinds of carbon atoms:
2
hybridized sp carbon (284.6 eV), carbon in the CvN or
–
C–OH group (286.0 eV), and carbon in the carboxyl or C–N
Fig. 3 HRTEM images (a–d) of NiFe@NC-3. The arrows point to some
tiny cracks on the NC layer surface.
3
5,36
group (288.1 eV).
Meanwhile, there are four contributions
in the N 1s spectrum (Fig. 5c), which are associated with four
2
different nitrogen species: nitrogen bonded to two sp carbon
atoms in the pyridine structure (398.5 eV), nitrogen bonded to
carbon as a linkage in the H–N–(C) (pyrrolic) structure (399.7
2
eV), nitrogen bonded to three carbon atoms within a graphene
Fig. 4 HAADF-STEM images (a, c) of representative NiFe@NC-3 with
the EDX mapping of C–K (b1), N–K (b2), Ni–K (b3) and Fe–K (b4); EDX
line scan profiles of Ni, Fe, C and N populations (d1–d4) along four red
lines in (c) over four single particles on the NiFe@NC-3 sample to show
the formation of NiFe alloy particles covered by the NC overlayer.
overlapping of Ni and Fe signals, as well as C and N signals. It
implies that Fe atoms dissolve homogeneously into Ni atoms,
forming an alloyed structure. Moreover, Ni and Fe line scan
spectra clearly show the same homogeneous distributions
along an individual particle (Fig. 4d). Interestingly, the distri-
butions of C and N species in the EDX scan line spectra
present a two-humped distribution at the edges of particles,
reflecting the formation of NC species in strong interfacial
contact with metal NPs. It further confirms that carbon and
nitrogen elements probably tend to distribute around NiFe
particles to form a thin NC layer.
Fig. 5 XPS survey spectrum (a), C 1s (b) and N 1s (c) for the NiFe@NC-3
sample.
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0
plane (400.8 eV), and oxidized nitrogen in the N–O group because surface Fe species are easily re-oxidized by air under
3
7–39
(
402.7 eV).
In the fine Ni 2p spectra of the NiFe@NC-x samples
the present synthesis conditions.
To investigate surface basicity, CO
2
-TPD of the experiments
(Fig. 6A), six fitted bands at about 853.0, 855.0, 860.8, 870.3, was conducted over the NiFe-3 and NiFe@NC-3 samples
8
2
72.5 and 879.6 eV are associated with the Ni 2p and Ni (Fig. 7). It is noted that there is no obvious CO desorption
3/2 2
0
2+
p
1/2 core levels of both metallic Ni and Ni species as well peak in the range of 200 to 750 °C for the NiFe-3 sample. In
2
+
40,41
as the satellite peaks from Ni species, respectively,
mainly because surface Ni species can be easily re-oxidized to tion peaks centred at about 376 °C, 473 °C and 627 °C, respect-
Ni species after preparation and exposure to air. It is noted ively, which are assigned to two kinds of medium-strength
that the binding energy for Ni species in NiFe@NC samples basic sites below 500 °C and strong basic sites above
contrast, the NiFe@NC-3 sample presents three CO
2
desorp-
0
2
+
0
4
4–47
shows a positive shift of about 0.6 eV, compared with the pre- 500 °C.
The fundamentally heightened surface basicity of
4
2
viously reported value of about 852.4 eV in the literature,
indicative of the electron transfer from NiFe NPs to the NC electron-rich nitrogen atoms in the NC layer.
layer, thus leading to the SMSI. As shown in Fig. 6B, there are As a result, thanks to the unique structure of NiFe-LDH pre-
the NiFe@NC-3 sample should originate from the presence of
4
8,49
three components fitted in the Fe 2p region of the NiFe@NC-2 cursors, as well as the surface NC overlayer, well-dispersed
and NiFe@NC-3 samples: a small peak at 706.4 eV corresponds NiFe alloy NPs can be generated in the NiFe@NC samples, and
0
2+
to metallic Fe , a sharp peak at 710.3 eV to Fe species and strong metal (NiFe alloy)–support (NC layer) interactions are
3
+
43
another one at 712.3 eV to Fe species. However, in the case formed at the core–shell interface. Moreover, the shell of the
of NiFe@NC-4, no metallic Fe species is detected, probably NC layer can also greatly enhance surface basicity.
0
3.3 Catalytic performance of catalysts
Commonly, o-CNB can be selectively hydrogenated to produce
5
0
o-CAN via multi-step reaction pathways. Herein, the hydrode-
chlorination of o-CNB as a side reaction can produce aniline
(
AN). In some cases, undesired o-chloronitrosobenzene (CNSB)
and N-chloro-phenylhydroxylamine (CPHA) intermediates can
be formed. To investigate the effect of interphase transport on
the hydrogenation, the agitation speed was employed from 300
to 900 rpm. Noticeably, the BA conversion over NiFe@NC-3
was unchanged at the stirring rate from 700 to 900 rpm
(
Fig. S3†). It demonstrates that the stirring rate of 900 rpm is
appropriate enough so that the hydrogenation rate is indepen-
dent of external mass transfer. Meanwhile, due to the small
size of the present actual catalyst powder ( < 10 μm), internal
(
or intraparticle) mass transport limitation can be ignored.
Fig. 6 XPS of Ni 2p (A) and Fe 2p (B) for NiFe@NC-2 (a), NiFe@NC-3 (b)
and NiFe@NC-4 (c) samples.
Fig. 7 CO
2
-TPD profiles for NiFe-3 and NiFe@NC-3 samples.
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Therefore, the intrinsic catalytic activity of catalysts can be sired by-products, and the hydrodehalogenation of haloge-
5
1
assessed under the present conditions.
nated nitroarenes often occurred. Obviously, the hydrogen-
Fig. 8 shows the change in the o-CNB conversion and the ation of nitroarene over NiFe@NC catalysts do not stop at
o-CAN selectivity over different catalysts with the reaction time intermediate stages. Also, all catalysts almost exhibit no
at 80 °C, while Table 2 summarises the catalytic results. activity in the dehalogenation side reaction, which is the same
Clearly, different catalysts present obvious differences in cata- as the previously reported results obtained using Fe
3 4
O –Ni cat-
lytic performance. Despite different catalysts, the o-CAN alysts in the hydrogenation of halogenated nitroarenes by
5
2
selectivity almost remains unchanged during the reaction. Gawande et al.
Notably, a quite high o-CNB yield of 99.5% is achieved over As listed in Table 2, among the tested catalysts, NiFe@NC-3
NiFe@NC-3 after a reaction time of 210 min. However, lower exhibits the highest initial rate for o-CNB conversion
o-CNB yields of 62.5 and 79.3% are obtained over NiFe@NC-2 (18.9 μmol gcat−1
s ) under milder reaction conditions (i.e.,
−1
and NiFe@NC-4, respectively, despite their high o-CAN selec- 0.5 MPa hydrogen pressure and 80 °C), which is nearly three
tivities (>99.0%). In contrast, the Ni@NC catalyst with the times that for the monometallic Ni@NC one, and even one-
same Ni loading as that of NiFe@NC-3 achieves a much lower order magnitude higher than that for the NiFe-LDH-derived
o-CAN yield of 24.9%. And, the NiFe-LDH-derived NiFe-3 cata- NiFe-3 catalyst. Moreover, the TOF value over the NiFe@NC-3
lyst, with two times Ni loading as that of NiFe@NC-3, only deli- catalyst (73.8 h− ) is larger than those over other catalysts. The
vers a low o-CAN yield of 7.3%. Such poor catalytic perform- lower TOF values for NiFe@NC-2 and NiFe@NC-4 are probably
ance of the NiFe-3 catalyst can be correlated with the presence associated with relatively low proportions of the NiFe alloy
of large-sized metallic NiFe alloy particles, as identified by the component, which are not beneficial for the activation of the
above XRD results. In both of the above cases, the o-CAN hydrogen molecule. As for the Ni@NC catalyst, the absence of
selectivity is slightly decreased to about 97.0%. Previously, it the NiFe alloy phase should contribute to its poor catalytic per-
was reported that the o-CNB hydrogenation easily stopped at formance. The above results illustrate the importance of Ni
several intermediate stages, thereby forming different unde- alloying with Fe for achieving high catalytic activity in the
1
Fig. 8 Changes in the conversion of o-CNB (A) and the selectivity to o-CAN (B) with the reaction time over catalysts in the o-CNB hydrogenation.
Reaction conditions: Hydrogen partial pressure, 0.5 MPa; reaction temperature, 80 °C.
a
Table 2 Catalytic performances of different catalysts in the o-CNB hydrogenation
Selectivity (%)
Initial rate (μmol gcat s−1
)
b
−1
TOF (h−1)
c
Yield of o-CAN (%)
Conv. (%)
o-CAN
AN
Others
d
Entry
Catalysts
1
2
3
4
5
NiFe@NC-2
NiFe@NC-3
NiFe@NC-4
Ni@NC
9.4
48.2
73.8
61.0
24.9
16.7
62.5
99.5
79.3
24.2
7.1
62.9
100
80.0
24.9
7.3
99.4
99.5
99.1
97.1
97.5
0.1
0.1
0.1
1.3
0.5
0.5
0.4
0.9
1.6
2.0
18.9
13.6
5.0
NiFe-3
1.7
a
b
Reaction conditions: Hydrogen partial pressure, 0.5 MPa; reaction time, 210 min, reaction temperature, 80 °C. Initial rate for o-CNB conver-
c
sion. TOF of o-CNB converted, which was given as the overall rate of o-CNB conversion normalized by the number of active sites determined by
H
d
2
pulse chemisorption within the initial 40 min. By-products mainly consist of CNSB and CPHA.
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8,59
o-CNB hydrogenation. Moreover, pure NC only delivers a very of metallic atoms and hydrogen spillover on the support.
low o-CAN yield of 1.1% in the o-CNB hydrogenation after 2 h. Compared with Fe-free Ni@NC, NiFe@NC samples yield larger
It suggests that the presence of metallic NiFe alloy species is a desorption areas, indicating that the NiFe alloy should have
crucial key governing the catalytic performance of NFe@NC stronger adsorption capacity of hydrogen because of the pres-
catalysts, and a single NC as a catalyst cannot contribute to the ence of the electronic effect between Ni–Fe atoms in NiFe alloy
catalytic activity. Interestingly, as illustrated in Table S1,† the NPs. Among the NiFe@NC samples, NiFe@NC-3 exhibits the
catalytic performance of the present NiFe@NC-3 catalyst in the highest adsorption capacity of hydrogen, indicative of the
o-CNB hydrogenation is better than or comparable with those enhanced ability for the dissociation of H . Meanwhile, we can
2
of other Ni-based catalysts reported previously in terms of the find that the adsorption capacity of hydrogen for NiFe-3 is
o-CNB (or p-CNB) conversion, the o-CAN (or p-CAN) selectivity extremely weak, almost zero, mainly due to the sintering of
and the rate of CNB conversion within a certain period, regard- metallic particles. Moreover, it is generally accepted that the
5
3–56
less of different experimental conditions.
spillover hydrogen on supports as active species, such as H*
+
+
60–62
Furthermore, to gain more information on the o-CNB atoms, radicals, H ions and H
3
ions,
can be involved in
hydrogenation over the NiFe@NC-3 catalyst, the kinetic curves hydrogenation reactions.
of the hydrogenation at different temperatures of 60 °C, 70 °C
In the present NiFe@NC-x catalyst system, due to the for-
and 80 °C were obtained. Here, the hydrogen pressure remains mation of the solid NiFe alloy core coated with a thin NC
unchanged during the reaction. Noticeably, −ln(1 − x) (x: layer shell with a thickness of 3–8 nm, there is no developed
o-CNB conversion) increases linearly with the reaction time internal micro- or mesoporous structure inside catalysts, and
(Fig. S4†), demonstrating that the rate of o-CNB hydrogenation the hydrogenation actually proceeds on the outer surface of
obeys a first-order reaction. Meanwhile, NiFe@NC-2 and catalysts, where o-CNB mainly is adsorbed on the NC layer
NiFe@NC-4 also present a similar kinetic character (Fig. S5†). shell and hydrogen is dissociated into active hydrogen
Furthermore, plotting ln k (k: reaction rate constant) vs. reci- species by exposed metallic species at the cracks of the NC
procal of temperature gives a lower apparent activation energy layer surface, followed by H-transfer from metallic NiFe NPs
−
1
(
E
a
) of about 38 kJ mol
for NiFe@NC-3 than those for up to the surface of the NC layer through the hydrogen spil-
−
1
−1
NiFe@NC-2 (73 kJ mol ) and NiFe@NC-4 (61 kJ mol
)
lover. In a way, the high catalytic efficiency of NiFe@NC cata-
(
Fig. S4†). Therefore, the lowered activation barrier of hydro- lysts is probably associated with the presence of a certain
6
3,64
genation evidently confirms the more obvious promotion degree of porosity inside the NC overlayer,
thus facilitat-
effect of the surface combination of NiFe NPs and the NC over- ing the penetration of hydrogen molecules into the thin NC
layer in the case of the NiFe@NC-3 catalyst. It implies that the layer and further activation of metallic sites to some extent.
present hydrogenation probably follows
Hinshelwood mechanism.
a
Langmuir– In this regard, an appropriate adsorption strength for reac-
5
7
tants, neither too weak nor too strong, is beneficial for the
To determine the nature of the interactions between the enhancement of the catalytic activity. For the as-fabricated
catalyst and hydrogen species, the catalysts were characterized NiFe@NC catalysts, surface nitrogen atoms in the NC layer
by H -TPD (Fig. 9). For all NiFe@NC catalysts, there is strong with strong basicity can interact with nitrogen atoms with a
2
hydrogen desorption in the range of 400–530 °C and broad de- partially positive change in the nitro group of o-CNB, which
sorption in the range of 550–700 °C, respectively, which are can induce an oriented chemisorption of o-CNB on the
2
associated with the release of chemisorbed H on the surface surface, resulting in the activation of the N–O bond to a large
6
5
extent, followed by the catalytic hydrogenation with the spil-
lover hydrogen from dissociated hydrogen species on active
NiFe sites. This kind of interaction between the nitro group
and the NC layer not only makes the nitro group easily reduci-
ble by dissociative hydrogen from active metallic species
through a spillover effect, but also effectively inhibits the
hydrodechlorination of the o-CNB and o-CAN product.
Correspondingly, we can speculate that selective o-CNB
hydrogenation over NiFe@NC catalysts may proceed accord-
ing to the following route: dissociated hydrogen species from
exposed metallic species at the cracks of the NC surface NiFe
alloy can spill over to the NC surface, where adsorbed o-CNB
is selectively hydrogenated to produce o-CAN. Therefore, the
present hydrogenation mainly is a surface reaction-controlled
reaction, and the internal diffusion of substrates inside cata-
lysts can be ignored.
Based on the structural characterization and catalytic test
studies, it is concluded that for NiFe@NC catalysts, exposed
metallic NiFe species at the defective NC layer may serve as
Fig. 9
2
H -TPD profiles of NiFe@NC-2 (a), NiFe@NC-3 (b), NiFe@NC-4
(
c), and Ni@NC (d) samples.
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activation centers for the hydrogen molecule coming from the
gas phase up to the surface of metallic NiFe NPs. Meanwhile,
the hydrogenation of o-CNB adsorbed on the surface of the NC
layer can occur through H-transfer from NiFe NPs. As a result,
the high catalytic efficiency of NiFe@NC-3 should be associ-
ated with both easier activation of hydrogen by NiFe alloy
species and favorable adsorption of o-CNB on the NC layer, as
well as the spillover hydrogen stemming from the novel core–
shell nanostructure. This is our hypothesis for the origin of
enhanced hydrogenation performance of the NiFe@NC-3 cata-
lyst. However, further investigation of the exact reason for the
difference in the catalytic performance of NiFe@NC catalysts
needs to be systematically explored through theoretical or
experimental methods in the future.
The reusability and recyclability of heterogeneous catalysts
are the two most important characters for industrial appli-
cations. Apart from the noble metal-free character of cata- Fig. 11 O -TPO profiles for NiFe-3 and NiFe@NC-3 samples.
2
lysts, excellent sustainability of the present NiFe@NC-3 cata-
lyst was further revealed in terms of easy recovery and re-
cycling properties. As shown in Fig. S6,† the magnetization
curve of NiFe@NC-3 shows a high magnetic saturation (Ms) structure of the core–shell nanostructure after recycling
−
1
value of about 41.4 emu g . When an external magnetic field seventeen times (Fig. S7†).
acts on the reaction solution with the catalyst after the reac-
Furthermore, the catalyst was characterized by O
-TPO
2
tion, NiFe@NC-3 can be quickly separated from the reaction experiments (Fig. 11). There is an oxygen consumption peak
solution and recovered magnetically, mirroring an excellent centred at 326 °C in the case of NiFe@NC-3, which is corre-
magnetic separation property of the catalyst. In the cycling lated with the adsorption of oxygen on the surface of the NC
2
tests, the NiFe@NC-3 catalyst was separated from the reaction layer. Correspondingly, a CO desorption peak appears at
solution under an external magnetic field. The recycling about 407 °C, which is associated with the oxidation of the
results are shown in Fig. 10. Notably, NiFe@NC-3 maintains carbon content in the NC layer. With increasing temperature,
its catalytic activity even after recycling seventeen times, no oxygen consumption can be detected, because the surface
2
along with a high o-CAN yield of 95.7% only decreased by NC shell can restrain the contact of O with the NiFe alloy
about 4%. Elemental analysis by ICP-AES reveals that the core and thus prevent metallic species from oxidation. In the
leaching losses of Ni and Fe species are almost negligible case of NiFe-3, however, the initial oxygen consumption
after seventeen cycles. The TEM image of the spent catalyst begins at 200 °C. There are two large oxygen consumption
revealed no observable change in the morphology and micro- peaks at higher temperatures of 422 °C and 550 °C, indicat-
ing much more oxygen consumption. The above results
reflect that NiFe NPs in NiFe@NC-3 exhibit excellent antioxi-
dation property, thanks to the structure of the NC overlayer,
which significantly prevents the sintering of NiFe NPs.
Thereupon, the as-fabricated NiFe@NC-3 catalyst possesses
exceptional stability in the o-CAN hydrogenation, which is
ascribed to the protection of the surface NC layer encapsulat-
ing inner NiFe alloy NPs and the strong core–shell inter-
actions, thus inhibiting the leaching, oxidation and aggrega-
tion of active species in the course of the reaction and
storage.
To investigate the feasibility of the NiFe@NC-3 catalyst,
selective hydrogenation of other substituted nitroarenes with
3
Cl, Br, I, OH, CH , and COOR functional groups was also eval-
uated. As shown in Table 3, NiFe@NC-3 also presents excellent
catalytic performance in the hydrogenation of other haloge-
nated aromatic nitro compounds, aromatic esters and pheno-
lic compounds, with high conversions (>90%) and selectivities
(
>90%) (entries 1–12). Notably, dehalogenation still is largely
avoided in all cases, and the yields of the corresponding aro-
matic amines reach as high as 90% or more under relatively
Fig. 10 Recyclability of the NiFe@NC-3 catalyst in the o-CNB
hydrogenation.
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a
Table 3 Catalytic performance of NiFe@NC-3 in the hydrogenation of other nitroarenes
Entry
1
Substrates
Products
T (°C)
Time (h)
3.5
Conv. (%)
100
Select. (%)
96.0
80
2
80
3
100
100
3
4
80
80
3.5
3.5
100
91.7
97.2
99.8
5
80
3.5
90.1
98.9
6
7
90
90
3.5
3.5
99.1
95.3
92.1
97.9
8
9
80
1
97.8
91.2
80
80
3.5
3.5
97.3
95.3
98.5
92.6
1
0
1
1
1
2
80
80
2.5
2.5
98.3
98.9
100
93.8
a
Reaction conditions: Nitroarene (0.64 mmol), catalyst (4.0 mg), ethanol (10 ml), and 0.5 MPa hydrogen pressure.
mild reaction conditions (i.e. low reaction temperature and low at least. Moreover, the unique NC shell, as well as strong core–
hydrogen pressure).
shell interactions, could contribute to excellent durability.
Such a NiFe@NC-3 catalyst also was highly active for the hydro-
genation of other aromatic nitro compounds (e.g. halogenated
aromatic nitro compounds, aromatic esters and phenolic com-
pounds) to produce commercially important aromatic amines.
Importantly, this present developed synthetic strategy is
general and extensible for designing and synthesizing other
cost-effective and stable noble metal-free, high-performing and
stable heterogeneous catalysts by changing the composition of
LDH precursors, which can be practically applied in a broad
range of eco-friendly and sustainable catalytic processes.
4
. Conclusions
In summary, we developed highly efficient, cost-effective and
magnetically recyclable core–shell structured NiFe@NC cata-
lysts via a facile in situ decomposition–reduction of the NiFe-
LDH/melamine mixture. The as-fabricated NiFe@NC-3 with a
Ni/Fe atomic ratio of 3.0 showed an exceptional catalytic hydro-
genation performance with a high o-CAN selectivity of 99.5%
upon complete conversion of o-CNB, which was much higher
than that of the catalyst derived from pure NiFe-LDH. It was
revealed that the NiFe alloy core and the NC overlayer shell
bearing strong basicity could benefit the adsorption and acti-
vation of reactants (i.e., hydrogen molecule and substrate). The
Conflicts of interest
catalyst could be recovered magnetically and reused 17 times There are no conflicts to declare.
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2
3 J. S. Lee, X. Q. Wang, H. M. Luo, G. A. Baker and S. Dai,
J. Am. Chem. Soc., 2009, 131, 4596–4597.
Acknowledgements
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National Natural Science Foundation of China (21776017;
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