Y. Ma et al. / Journal of Catalysis 377 (2019) 174–182
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adjustable elemental composition, and uniform nanoscale size
have shown potential application prospects in the design of inter-
facial catalysts [39]. Recently, some nanostructured molybdenum
carbide and phosphide/metal hybrid materials have been prepared
from POM-based precursors. All of them exhibited remarkable
hydrogen evolution reaction (HER) performance due to their speci-
fic interfacial effects [40–42]. These concepts for the design of
multi-interfacial catalysts via polyoxometalates impel us to
explore highly effective catalysts for selective reduction of nitroar-
enes. Here, we demonstrate that nickel–tungsten carbide compos-
ite nanoparticles loaded on carbon (denoted as Ni-WC/C) can act as
an efficient catalyst for the selective reduction of nitroarenes by
hydrazine hydrate (N2H4ÁH2O) under mild conditions. In a typical
experiment with the Ni-WC/C catalyst, nitrobenzene (NB) can be
highly selectively reduced to AN or HAB just by adjusting the molar
ratio of NB to N2H4ÁH2O. For example, when the molar ratio of NB
to N2H4ÁH2O was 1:13, NB was directly reduced to AN with a yield
of 98%. This result is comparable to those obtained with commer-
cial 20% Pt/C and 5% Pd/C. When the molar ratio of NB to N2H4ÁH2O
was modulated to 8:13, HAB became the main product with a yield
of 94%. This unexpected switchable catalytic reaction with high
activity and selectivity arises from the remarkable synergistic role
between Ni and the WC center in the Ni-WC/C catalyst. Density
functional theory (DFT) calculation results suggest that the multi-
ple interfacial structure can optimize the electronic structure of the
Ni surface, further dominating the final reduced products.
mixtures were identified by GC–MS and 1H NMR, and the conver-
sion of nitrobenzene and product yields were analyzed by a gas
chromatograph (Ang) equipped with a flame ionization detector
(FID) and a capillary column (J & W DB-WAX, 30 m  0.32 mm)
with nonane as an internal standard. The final yield of product is
calculated based on the theoretical value of the full conversion.
The recyclability of the Ni-WC/C catalyst was also investigated
under the above reaction conditions. Taking the selective hydro-
genation of NB to AN as an example, 0.5 mmol of nitrobenzene,
400 ll of N2H4 H2O (80 wt%), and 10 mg of catalyst were added
to 2 ml of C2H5OH and stirred at 60 °C for 5 min. The reaction time
for the recycling experiment that selectively hydrogenates NB to
HAB is 1 h. After the reaction, the catalyst was recovered by cen-
trifugation, washed several times with anhydrous ethanol, and
then dried for the next cycle.
2.5. Theoretical calculation methods
Periodic DFT calculations were performed using the Vienna
ab initio Simulation Package (VASP) with exchange and correlation
potential represented by the PBE approximation [44,45]. A suitable
k-point grid of 3 Â 3 Â 1 was generated with the Monkhorst–Pack
algorithm [46]. The kinetic energy cutoff was set to 400 eV. Results
were obtained until the forces and energy were less than
0.03 eV ÅÀ1 and 10À5 eV, respectively.
The parent WC (0 0 1) and Ni (1 1 1) surfaces were simulated by
4 Â 4 and 5 Â 5 unit cells with three WC and Ni layers, respec-
tively. A vacuum larger than 15 Å perpendicular to the surface
avoided interaction between repeated cells. The topmost tung-
sten–carbon or Ni layer was relaxed, while the remaining layers
were frozen. The Ni-WC composite system was constructed with
one layer of Ni (1 1 1) atoms adsorbed on top of three WC (0 0 1)
layers. During the optimization, the adsorbed species, the Ni layer,
and the top WC layer were kept relaxed.
2. Experimental
2.1. Chemicals and materials
Dicyanamide (DCA), benzene-1,3,5 tricarboxylic acid (H3BTC,
98%), nickel chloride hexahydrate (NiCl2Á6H2O), tris(hydroxyme
thyl)aminomethane hydrochloride (TrisÁHCl), and H3PW12O40ÁnH2O
were purchased from Aladdin Industrial Company. Commercial 20%
Pt/C and 5% Pd/C catalysts were purchased from Alfa Aesar China
(Tianjin) Company. All chemicals were used as received without
3. Results and discussion
further purification. [Ni(en)2(H2O)2]6{Ni6(Tris)(en)3(BTC)1.5(B-a-
3.1. Synthesis and characteristics of Ni-WC/C
PW9O34)}8Á12enÁ54H2O (abbreviated as Ni54W72) was synthesized
according to the method reported by Yang et al. [43].
The nanoscale composite catalyst Ni-WC/C was fabricated via a
one-step method according to our previous work [39] with slight
modifications by pyrolyzing a mixture of [Ni(en)2(H2O)2]6{Ni6(-
2.2. Synthesis of Ni-WC/C
Tris)(en)3(BTC)1.5(B-
a
-PW9O34)}8Á12enÁ54H2O [43] (denoted as
The synthesis of Ni-WC/C was according to our previous work
[39] with slight modifications, as follows: 0.2 g Ni54W72 and 0.1 g
DCA were placed in an agate mortar and then absolutely mixed
by ball milling. Afterward, the powder was pyrolyzed in a pipe fur-
nace at 500 °C for 30 min under N2 with a heating rate of 2 °C/min,
and then further heated at 650 °C for 6 h with a heating rate of
5 °C/min. The as-obtained sample is denoted as Ni-WC/C.
Ni54W72) and dicyandiamide (DCA) at 650 °C in N2 (Fig. 1a). TEM
and SEM images in Figs. 1 and S1 in the Supporting Information
show the morphology and superstructure of as-made Ni-WC/C
material composed of Ni-WC hybrid nanoparticles (NPs) with aver-
age size ca. 8 nm scattered on porous carbon sheets (Fig. S2). The
high-resolution (HR) TEM image in Fig. 1c exhibits intimate con-
tact between Ni and WC, forming unique Ni/WC interfaces. The lat-
tice fringes with interplanar spacings 0.25 and 0.18 nm correspond
to the (1 0 0) planes of hexagonal WC and the (2 0 0) planes of
metallic Ni, respectively. The bright-field scanning TEM (BF-
STEM) images, high-angle annular dark-filed scanning TEM
(HAADF-STEM) images, and corresponding elemental mapping
images (Fig. 1e–i) of an individual Ni-WC nanoparticle demon-
strate that Ni and W distribute evenly on the surface of Ni-WC
nanoparticles, further affirming the presence of Ni/WC interfaces
(Fig. S3). EDX results also validate the elemental composition of
Ni-WC/C. The content of Ni-WC in Ni-WC/C is almost 58.49 wt%
(Tables S1 and S2).
2.3. The procedure for decomposition of hydrazine hydrate
In a typical case, 2 ml of C2H5OH and 10 mg of catalyst were
added to a hermetic Dewar flask. Under magnetic stirring, 200 ll
of N2H4ÁH2O (80 wt%) was injected into the system and heated at
60 °C for 4 h. An aliquot of gaseous product was periodically
removed from the flask. The identity of products and the yield of
hydrogen were ascertained by gas chromatography (GC).
2.4. The procedure for selective hydrogenation of nitrobenzene
The phase composition of Ni-WC/C was further analyzed by
powder X-ray diffraction patterns (PXRD), illustrated in Fig. 2a.
The characteristic peaks located at 31.57°, 35.88°, 48.62° and
64.51° can be indexed to the (0 0 1), (1 0 0), (1 0 1), and (1 1 0)
In a typical case, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, or 4 mmol of nitroben-
zene (NB), 400
ll of N2H4 H2O (80 wt%), and 10 mg of catalyst were
added to 2 ml of C2H5OH and stirred at 60 °C. After the reaction, the