Highly dispersed nanocrystallines WC supported on microwave exfoliated RGO by ionic liquid and its catalytic performance in electroreduction of nitrobenzene

Article abstract of DOI:10.1016/j.cattod.2012.08.017

Tungsten carbide (WC) nanocrystallines were loaded onto the surface of microwave-exfoliated reduced graphene oxide (RGO) using program-controlled reduction-carbonization technique and modified impregnation method in the presence of ionic liquid (IL). The characterization results show that WC nanocrystallines with a size of ca.80 nm were highly dispersed on RGO with the assistance of IL. The electrocatalytic activity of the prepared WC/RGO (IL) was evaluated in the nitrobenzene reduction reaction. It was found that the WC/RGO (IL) gave better activity and stability than single RGO or WC/RGO that were prepared in the absence of IL. The high activity of WC/RGO (IL) can be explained to be the result of the synergistic function between WC and RGO. It was also proved that the pre-treatment of GO by microwave-exfoliated method was an effective way to form the graphene-based intercalation materials. Our study demonstrated that WC/RGO (IL) could be a promising alternative catalyst toward the nitrobenzene electroreduction because of its higher performance and lower cost.

Full text of DOI:10.1016/j.cattod.2012.08.017

Catalysis Today 200 (2013) 87–93
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Highly dispersed nanocrystallines WC supported on microwave exfoliated RGO
by ionic liquid and its catalytic performance in electroreduction of nitrobenzene
∗
Weiming Liu, Meiqin Shi, Xiaoling Lang, Di Zhao, Youqun Chu, Chun’an Ma
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, School of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou
310014, Zhejiang, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 April 2012
Received in revised form 9 August 2012
Accepted 16 August 2012
Available online 18 September 2012
Tungsten carbide (WC) nanocrystallines were loaded onto the surface of microwave-exfoliated reduced
graphene oxide (RGO) using program-controlled reduction–carbonization technique and modified
impregnation method in the presence of ionic liquid (IL). The characterization results show that WC
nanocrystallines with a size of ca.80 nm were highly dispersed on RGO with the assistance of IL. The
electrocatalytic activity of the prepared WC/RGO (IL) was evaluated in the nitrobenzene reduction reac-
tion. It was found that the WC/RGO (IL) gave better activity and stability than single RGO or WC/RGO
that were prepared in the absence of IL. The high activity of WC/RGO (IL) can be explained to be the
result of the synergistic function between WC and RGO. It was also proved that the pre-treatment of GO
by microwave-exfoliated method was an effective way to form the graphene-based intercalation mate-
rials. Our study demonstrated that WC/RGO (IL) could be a promising alternative catalyst toward the
nitrobenzene electroreduction because of its higher performance and lower cost.
Keywords:
Tungsten carbide
Reduced graphene oxide
Ionic liquid
Microwave
Nitrobenzene
Electroreduction
© 2012 Elsevier B.V. All rights reserved.
1
. Introduction
Nitrobenzene is widely used in the chemical production, such
also expected to show better performance toward the nitroben-
zene electroreduction. In our early work, we investigated WC
and WC/CNTs electrodes in the nitrobenzene electroreduction.
The results proved that WC had the good electrocatalytic activity
toward nitrobenzene reduction. Moreover, WC/CNTs showed bet-
ter performance in comparison with the case of pure WC or CNTs,
which is ascribed to the superior surface effect of CNTs and good
dispersion of WC on CNTs. Hence, more inspiring results on WC
composites can be achieved if the suitable supports and effective
synthesis ways are found to enhance the entire distribution of WC
on the support and improve its electronic property.
Graphene, a single-atom-thick planar sheet of hexagonally
arrayed sp2 hybridized carbon, which is the basis of carbon
nanotubes and graphite, has attracted tremendous attention in
nanoscience because of its strictly two-dimension and its out-
standing electronic, thermal and mechanical properties [10–13].
It exhibits promising potential application in many technological
fields, such as nanoelectronics, sensors, nanocomposites, batter-
ies, supercapacitors and hydrogen storage [10], especially as a
new catalyst support to enhance electrocatalytic activity [14–17].
At present, chemically converted graphene, which is prepared
typically from graphite oxide (GO) or graphene oxide (GO) with dif-
ferent reductants, is commonly named as reduced graphene oxide
as aniline, aniline dyes, explosives, pesticides and drugs, and also
acts as a solvent in products like paints, shoes and floor metal
polishes [1]. However, nitrobenzene has been understood as a
great threat to human health and environment because of its high
toxicity [2,3]. The strong electron affinity of nitro reduces the elec-
tron cloud density of benzene ring and thus makes nitrobenzene
very stable. It is urgent to explore the efficient ways to degrade
nitrobenzene. Nitrobenzene electroreduction has recently aroused
increasing interest because of its cleaner process [4], which plays
an important role in industries. Therefore, the novel cathodic elec-
trocatalytic material with better activity and lower cost for the
electroreduction of nitrobenzene is highly desirable.
In the past two decades, our group has been studying WC and its
composites because of its “platinum-like” behavior in surface catal-
ysis [5] and its strong resistance to corrosion and catalytic poisons.
WC has lower hydrogen overpotential and offers stronger hydrogen
adsorption capacity which means to be the suitable electrocata-
lyst in the electroreduction. Additionally, WC was reported as a
promising material to replace noble catalyst (such as platinum) in
alkane reforming and alkane isomerization [6–9]. Therefore, WC is
(RGO). RGO is superior compared to CNTs, such as lower produc-
tion cost, higher available surface area, and being easily obtained
by chemical conversion of inexpensive GO [14]. Meanwhile,
besides being the support, RGO also can be used as a catalyst for
∗ Corresponding author. Tel.: +86 571 88320830; fax: +86 571 88320830.
E-mail address: science@zjut.edu.cn (C. Ma).
0
920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.08.017

8
8
W. Liu et al. / Catalysis Today 200 (2013) 87–93
hydrogenation of nitrobenzene [18]. Therefore, much higher
electrocatalytic activity can, in principle, be achieved by the intro-
duction of WC onto a RGO support.
carbonization and WC/RGO synthesized in the absence of ionic liq-
uid were also prepared, denoted as RGO and WC/RGO respectively.
In this study, precursor GO was exfoliated by microwave irra-
diation to obtain the high-specific area support. WC/RGO (IL)
was prepared by depositing highly dispersed WC nanocrystallines
on the support using program-controlled reduction–carbonization
technique and modified impregnation method in the presence
of ionic liquid (IL). Compared with other common processes, the
procedure used in our study for preparing the WC/RGO (IL) has
several advantages: (1) under microwave irradiation, the pristine
GO powders were expanded and exploded rapidly to result in a
porous structure and larger interlayer spaces, which constituted
a framework for tungsten-containing groups to infiltrate into the
interlayers of GO; (2) ethanol was used as the solvent, and GO
can be easily dispersed in ethanol because of its hydrophilicity;
2
.4. Sample characterization
N2 adsorption–desorption was examined by ASAP 2020
(Micromeritics, America) and the specific surface area of GO was
measured by Brunauer–Emmett–Teller (BET) method. The mor-
phologies of the samples were observed by SEM Hitachi S-4700
II (Hitachi, Japan), using Cu K˛ radiation (ꢀ = 0.154 nm). The crys-
talline structures were investigated by X-ray diffraction (XRD) with
an X’Pert PRO X-ray (PANalytical, Netherlands) at room tempera-
ture. Cu K˛ radiation (40 kV, 40 mA) was applied and an angle range
from 10 to 80 was recorded at 0.05 increments. Transmission
electron microscopy (TEM) was performed with a Tecnai G2 F30
S-Twin microscope (FEI, Netherlands), coupled with energy disper-
sive X-ray spectrometer (EDX, Thermo NORAN VANSTAGE ESI), also
using Cu K˛ radiation. The element distribution of the powders was
characterized by EDX.
◦
◦
(
3) in WCl , the source of WC in our processes, W end of the
6
bond is positive and the Cl end is negative because Cl is more
electronegative than W, while the one end of IL ([BMIM][PF6])
−
is [PF6] which can bond with atom W. On the other hand, the
+
another end of IL, [BMIM] , has the tendency to move forward to
2.5. Electrochemical measurements
the interlayers of GO which are negatively charged due to the oxy-
genated groups such as hydroxyl and carboxyl groups. In this way,
the tungsten-containing groups will be anchored tightly on the
surfaces of GO. As discussed above, the presence of IL facilitated
the self-assemble process to obtain the highly dispersed precursor
tungsten-containing groups/GO.
The electrochemical measurement of the samples was per-
formed in a conventional three electrode system. The reference
electrode, counter electrode, and working electrode were satu-
rated calomel electrode (SCE), Pt plate (2 cm ) and glassy carbon
2
disk (3 mm in diameter), respectively. All the electrochemical
experiments were carried out using the CHI 660d electrochemical
workstation (Cheng-Hua, Shanghai, China) at room temperature.
The working electrodes were composed of the WC/RGO (IL) pow-
ders deposited as an ultra-thin layer on an activated glassy carbon
disk, 5 mm diameter. WC/RGO (IL) was suspended in ethanol con-
taining 5 wt% Nafion solution (Ruibang New Energy Technology
Co., Ltd., China) under ultrasonication for 30 min. A 20 ␮L of the
suspension was pipette on top of the glassy carbon disk electrode
and then dried in infrared desiccators to form the ultra-thin layer.
WC/RGO and RGO electrodes were prepared by the same process.
2
. Experimental
2.1. Synthesis of graphite oxide (GO)
GO was derived from graphite powders (SP, from sinopharm
chemical reagent Co., Ltd.) with a modified Hummers method
which was originally presented by Kovtyukhova et al. [19,20].
Graphite and KMnO powders were mixed in concentrated H SO
4
2
4
◦
(
23 mL) under constant stirring at ca. 10 C for 2 h. After the mixture
◦
was stirred at ca. 35 C for 30 min, deionized (DI) water was slowly
added to the beaker and the oxidation reaction proceeded at ca.
−1
The aqueous solutions containing 0.5 mol L H SO with or with-
2
4
−1
out 5 mmol L nitrobenzene were used as electrolytes, which were
purged with highly purified nitrogen for 30 min prior to electro-
chemical measurement, and all experiments were carried out at
room temperature.
◦
9
8 C for 1 h. The reaction was ended by adding DI water followed
by 30% H O solution so as to neutralize redundant KMnO . Finally,
the mixture was centrifugated and washed with DI water several
times, and then the obtained mixture was dried in an air oven at
2
2
4
◦
8
0 C.
3. Results and discussion
2.2. Pretreatment of GO
3.1. Exfoliation of GO by microwave irradiation
The as-prepared GO powders were sealed in a microwave kettle
GO was exfoliated by microwave irradiation. Fig. 2(a) and (b)
and treated in a microwave apparatus Initiator EXP EU (Biotage,
are N2 adsorption–desorption isotherms of GO before and after
microwave treatment, obviously both the samples are character-
ized by type II isotherms and possess hysteresis loops of type
H3. According to the prescription of IUPAC, the type II isotherm
indicates that there are no micropores or large mesopores in the
samples and the type H3 hysteresis loop at relatively high pres-
sure suggests that there exist asymmetrically slit-shaped pores
with large pore size. Nevertheless, it is interesting to note that after
◦
Sweden) in ambient conditions at 110 C.
2
.3. Synthesis of WC/RGO
Solution A: tungsten hexachloride (WCl ) was added to a beaker,
6
mixed with 1 mL of [BMIM][PF6] (Lanzhou Greenchem ILS, LICP.
CAS. China) and 40 mL of ethanol. Solution B: after microwave irra-
diation, the GO powders were dispersed in ethanol. Subsequently,
solution A was added to solution B and the mixture was heated
microwave treatment the adsorption of N on GO, is enhanced com-
2
paring Fig. 2(b) with (a) at the same P/P , and the BET values of the
0
◦
2
to remove the ethanol, then centrifugated and dried at 70 C to
samples are 13.6 and 79.8 m /g respectively as seen in Fig. 2(a)
form a precursor. The as prepared precursor was carbonized in a
tubal resistance furnace. Firstly, the furnace was purified with N2
for 30 min to remove the air. The obtained precursor was subse-
and (b). Under microwave irradiation, the pristine GO powders
which were heated in short time expanded and exploded rapidly.
A great amount of large pores were constituted because of the
release of adsorbed water existing in interlayers and partial decom-
position of the oxygen-containing groups (epoxide, hydroxyl,
carboxyl, carbonyl groups). Thus after microwave exfoliation, the
◦
quently preserved at 900 C for a few hours with the gas of CO
and H . The fabrication procedure of WC/RGO was demonstrated in
2
Fig. 1. For comparison, RGO without loading WC obtained by direct

W. Liu et al. / Catalysis Today 200 (2013) 87–93
89
Fig. 1. Schematic illustration shows synthesis of WC/RGO (IL) composites.
specific surface area of GO was increased compared with the sam-
3.2. Characterization of catalysts
Fig. 4 gives the XRD patterns of the three samples. The peaks at
ples prepared by normal Hummers method, which is the reason
why the microwave pretreated GO possessed higher adsorption
quantity.
◦
◦
the 2ꢁ of 26.53 and 43.06 with the d-spacing values of 3.36 and
Fig. 3 is the SEM images showing the difference of the morpholo-
gies of GO before and after microwave irradiation. In Fig. 3(a), GO
obtained by the normal Hummers method shows a tightly stack
between graphite layers, while GO powders after microwave irra-
diation in Fig. 3(b) were exfoliated to a very large extent and has
2.10 are corresponding to the (0 0 2) and (1 0 0) facets of RGO [17],
◦
◦
◦
◦
◦
◦
◦
the 2ꢁ of 31.36 , 35.53 , 48.23 , 64.14 , 65.56 , 73.06 , 75.06 and
◦
76.93 with the d-spacing values of 2.85, 2.52, 2.28, 1.45, 1.42, 1.29,
1.26 and 2.10 are corresponding to the (0 0 1), (1 0 0), (1 0 1), (1 1 0),
(0 0 2), (1 1 1), (2 0 0) and (1 0 2) facets of WC [26–28]. As shown in
Fig. 4b, the typical diffraction peak (0 0 1) of GO disappears while
the typical diffraction peak (0 0 2) of RGO enhances after reduc-
“
a wave-like” morphology [21–24]. Upon microwave irradiation,
the volume and interlayer space of the GO powders expand rapidly
because of ‘violent fuming’ [25], thereby causes a larger specific
surface area which matches with the BET results. Moreover, the
increase of the interlayer space is applicable to the total immer-
sion of GO in the solvent and the subsequent assembling of active
components.
tion by H . It could be concluded that GO sheets were effectively
2
reduced to graphene and restacked into an ordered graphite crys-
talline structure via program-controlled reduction–carbonization
technique. In Fig. 4(a) and (b), the three typical diffraction
peak (0 0 1), (1 0 0), and (1 0 1) of WC can be observed, which
Fig. 2. N2 adsorption–desorption isotherms of GO (a) before and (b) after treatment by microwave irradiation.

9
0
W. Liu et al. / Catalysis Today 200 (2013) 87–93
Fig. 3. SEM images of GO (a) before (b) after microwave irradiation.
the images of RGO supported WC nanocrystallines synthesized
with and without IL. It can be seen in Fig. 5(a) that the WC
nanocrystallines, the bright particles, in the sample of WC/RGO
aggregate together and distribute mainly on the edges of RGO
sheets with a size of around 130 nm. In Fig. 5(b), different
from the previous figure, the WC nanocrystallines in the sam-
ple of WC/RGO (IL) deposited on both sides and edges of RGO
sheets with a smaller size and higher dispersion. This could
be attributed to the use of IL in the synthesis progress. WO
species, generated from WCl , can anchor on the interlayers of
6
RGO uniformly with the existence of IL. So that the nucleation
of nanocrystallines was dispersed efficiently and the growing
of crystallines was also hindered because the interaction of
IL will make great impact on the nucleation and growth of
nanocrystallines [34].
Fig. 6 shows the typical TEM images and the correspond-
ing EDX patterns of WC/RGO and WC/RGO (IL). The similar
results were confirmed in the aspect of distribution and parti-
cles size, which suggested that the IL played an important role
in controlling particle size of WC and preventing them from
aggregation.
Fig. 4. The XRD patterns of (a) WC/RGO, (b) WC/RGO (IL) and (c) RGO.
indicates that WC nanocrystallines were loaded on RGO success-
fully. However, a small quantity of W C can also be found in
2
both samples, probably because of the incomplete carbonization
of WCl6.
3.3. Electrochemical measurements of the samples
Ionic liquid (IL), a kind of room temperature organic salts,
has been widely used as a new reaction medium due to its
unique properties such as low volatility, good thermal stability,
good dissolving ability, high ion conductivity and wide electro-
chemical window [29–33]. In this work we prepared WC/RGO
The electrocatalytic activity of WC/RGO, WC/RGO (IL) and RGO
electrodes toward nitrobenzene electroreduction were evaluated
−
1
−1
in 0.5 mol L H SO with 5 mmol L nitrobenzene using CV, as
2
4
shown in Fig. 7. It was reported [4,35] that the procedure of the
nitrobenzene electroreduction reaction in acid medium could be
expressed as follows.
(
IL) in the presence of IL ([BMIM][PF6]). Fig. 5(a) and (b) are
Fig. 5. SEM images of GO (a) WC/RGO and (b) WC/RGO (IL). The insets are their SEM images in high magnification.

W. Liu et al. / Catalysis Today 200 (2013) 87–93
91
Fig. 6. TEM image of GO (a) WC/RGO and (b) WC/RGO (IL). The corresponding EDX patterns of (c) WC/RGO and (d) WC/RGO (IL).
when the three prepared samples were tested in 0.5 mol L 1 H SO .
−
2
4
After adding the nitrobenzene into H SO , the irreversible cathodic
2
4
current peaks appeared at −0.345 V, −0.333 V and −0.334 V respec-
tively in the three samples, and the corresponding data are listed
in Table 1. Compared with those of WC/RGO and RGO, the cathodic
peak potential of WC/RGO (IL) shifted positively for ca.12 mV and
1
mV. Furthermore, it can be observed from Fig. 7 and Table 1 that
the onset potentials of nitrobenzene electroreduction on WC/RGO
(IL) are more positive than those on WC/RGO and RGO.
The catalytic currents are calculated for per unit mass which
refers to the total film on GC surface. The currents observed on
CV curves of WC/RGO and WC/RGO (IL) are 1.64 and 1.95 mA/mg,
higher than the value of 0.34 mA/mg, obtained from RGO (Table 1).
Based on Randles–Sevcik equation, the peak currents are propor-
tional to the area of the electroactive surface area [3]. So the
electroactive surface area for WC/RGO (IL) electrode is larger than
those of the other two. According to early works, RGO can be used
as a catalyst for hydrogenation of nitrobenzene [21]. The change
The nitrobenzene first diffuses onto the electrode from the
solution and is reduced to phenylhydroxylamine (PHA) by the four-
electron at the potential range of −0.6 to −0.2 V (reaction (1)).
Consequently PHA continues to be reduced to aniline by the two-
electron at the potential of ca. −0.8 V (reaction (2)) or diffuses into
the solution to produce p-aminophenol by rearrangement (reac-
tion (3)). Compared with the literature data, the main reaction in
our test is the reduction of nitrobenzene to PHA corresponding to
the main reduction peak at ca. −0.35 V in Fig. 7. The inset of Fig. 7,
is the CV curves of the WC/RGO, WC/RGO (IL) and RGO electrodes
Table 1
Comparison of different electrocatalytic activity parameters from the cyclic voltam-
metry of nitrobenzene electrochemical reduction among the prepared WC/RGO,
WC/RGO (IL) and RGO catalysts.
ip a (mA/mg mass)
Ep b (V vs. SCE)
Samples
Onset potential
(V vs. SCE)
WC/RGO (IL)
WC/RGO
RGO
1.95
1.64
0.34
−0.333
−0.345
−0.334
−0.243
−0.261
−0.246
−1
in 0.5 mol L H SO , provides the background information for the
2
4
electrochemical processes taking place on the catalysts surfaces.
There is no current peak except for the hydrogen evolution peaks
a
Peak current.
Peak potential.
b

9
2
W. Liu et al. / Catalysis Today 200 (2013) 87–93
Fig. 7. The cylic voltammograms (CV) of (a) WC/RGO, (b) WC/RGO (IL) and (c) RGO
Fig. 9. The cylic voltammograms (CV) of (a) WC/RGO, (b) WC/RGO (IL) and (c) RGO
−1
−1
electrocatalysts tests in 0.5 mol L H2SO4 + 5 mmol L nitrobenzene at a scan rate
−
1
−1
electrocatalysts tests in 0.5 mol L H2SO4 + 5 mmol L nitrobenzene at a scan rate
−
1
of 20 mV s at room temperature. The inset is the CV curves of the same electro-
−
1
of 20 mV s at room temperature after 400 cycles.
−1
catalysts in 0.5 mol L H2SO4 solution.
Table 2
in the current density demonstrates that the improved reaction
kinetics of nitrobenzene reduction on WC/RGO (IL) and WC/RGO
catalysts may due to synergistic effect between WC and RGO. The
results above indicate that all the three samples exhibit electrocat-
alytic activity toward the reduction of nitrobenzene, but WC/RGO
Comparison of different electrocatalytic activity parameters from the cyclic voltam-
metry of nitrobenzene electrochemical reduction after 400 cycles between the
prepared WC/RGO, WC/RGO (IL) and RGO catalysts.
ip a (mA/mg mass)
Ep b (V vs. SCE)
Samples
WC/RGO (IL)
WC/RGO
RGO
1.45
1.34
0.28
−0.329
−0.349
−0.346
(
IL) electrode performs best. The greatly enhanced electrochemical
behavior of WC/RGO (IL) may be attributed to its smaller size and
more uniformly dispersed on the surfaces of RGO.
a
Peak current.
Peak potential.
b
Fig. 8 presents the Linear Sweep Voltammetry (LSV) of WC/RGO
−1
−1
(
IL) electrode in 0.5 mol L
H SO + 5 mmol L nitrobenzene at
2
4
different scan rate at room temperature. The plot of peak cur-
rents vs. square root of scan rate is given in the inset. A
nearly linear dependence is found on the WC/RGO (IL) electrode
catalyst is the most positive value, compared with those of the
other two. The results reconfirmed that toward the nitrobenzene
electroreduction, WC/RGO (IL) gave the best performance.
(
Fig. 8), suggesting that the electrode reaction is controlled by the
diffusion.
The CV curves of the same samples after 400 cycles are shown
4. Conclusions
in Fig. 9. It is clear that all the peak current densities decrease for
nitrobenzene reduction on three catalysts. However, WC/RGO (IL)
still illustrates the highest current peak and the results are listed
in Table 2. Moreover, both before and after 400 cycles, the onset
potential of the nitrobenzene electroreduction on WC/RGO (IL)
In summary, WC/RGO (IL) was successfully synthesized using
program-controlled reduction–carbonization technique and mod-
ified impregnation method in the presence of ionic liquid (IL). IL
was used as dispersing agent to prepare the WC nanocrystallines
with smaller size and better distribution. Compared to the RGO
and WC/RGO catalyst, the WC/RGO (IL) displayed better stability
and higher electrocatalytic activity toward nitrobenzene reduction
reaction, showing a significantly lower overpotential and a higher
peak current. The WC/RGO (IL) catalyst could be a promising alter-
native catalyst for nitrobenzene electroreduction because of its
higher activity and lower preparing cost. Meanwhile, our method
used in this work for preparing WC/RGO (IL) also set an example
for synthesis of other carbide compounds supported on RGO.
Acknowledgement
This work was supported by International Science & Technology
Cooperation Program of China (2010DFB63680), Key Project of Nat-
ural Science Foundation of Zhejiang Province (Z4100790), Science
and Technology Program of Zhejiang Province (2011R09002-08),
the 973 Project from the Ministry of Science and Technology of
China (2012CB722604), and Science and Technology Major Project
in International Cooperation of Zhejiang Province (2008C14040).
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Fig. 8. The Linear Sweep Voltammetry (LSV) of WC/RGO (IL) electrocatalysts tests
in 0.5 mol L H2SO4 + 5 mmol L nitrobenzene at different scan rate at room tem-
perature. The inset is the plot of peak currents vs. square root of scan rate.
−
1
−1
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