Low-cost CuNi-CeO2/rGO as an efficient catalyst for hydrolysis of ammonia borane and tandem reduction of 4-nitrophenol

Article abstract of DOI:10.1016/j.jallcom.2017.09.075

Low-cost metallic nanoparticles as catalysts with good performance are highly desired for the development of hydrogen storage materials. In this work, ceria-doped CuNi nanoparticles were successfully immobilized on the surfaces of reduced graphene oxide (rGO) by a simple co-reduction approach in aqueous solution at room temperature, characterized by ICP-OES, XRD, TEM, SEM-EDS and XPS techniques. The as-synthesized CuNi-CeO₂/rGO nanocomposites with various metal contents had been employed as heterogeneous catalysts for the hydrolytic dehydrogenation of ammonia borane (NH₃BH₃, AB) under mild conditions. The optimal catalyst of Cu_0.8Ni_0.2-CeO₂/rGO with CeO₂ content of 13.9 mol% exhibited an excellent catalytic activity with the exceedingly high TOF value of 34.4·mol_H2·mol_catalyst^?1·min^?1 at 298 K. The apparent activation energy was calculated to be 19.1 kJ mol^?1, even lower than most of noble-metal-based catalysts for AB dehydrogenation. The detailed kinetics of AB hydrolysis catalyzed by Cu_0.8Ni_0.2-CeO₂/rGO had been investigated on different concentrations of catalyst, substrate and temperatures. Moreover, the obtained in situ hydrogen from AB hydrolysis can be further applied to reduce 4-nitrophenol with a high TOF value of 8.1 min^?1 at room temperature. The high catalytic activity of Cu_0.8Ni_0.2-CeO₂/rGO would be contributed to the small size of CuNi-CeO₂ NPs, metal-metal synergy, and the metal-support interaction.

Full text of DOI:10.1016/j.jallcom.2017.09.075

Journal of Alloys and Compounds 728 (2017) 902e909
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Low-cost CuNi-CeO /rGO as an efficient catalyst for hydrolysis of
2
ammonia borane and tandem reduction of 4-nitrophenol
*
Ying-Hua Zhou , Suqin Wang, Yu Wan, Jingjing Liang, Yu Chen, Shizhong Luo, Cheng Yong
The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials (State Key Laboratory Cultivation
Base), College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2017
Received in revised form
Low-cost metallic nanoparticles as catalysts with good performance are highly desired for the devel-
opment of hydrogen storage materials. In this work, ceria-doped CuNi nanoparticles were successfully
immobilized on the surfaces of reduced graphene oxide (rGO) by a simple co-reduction approach in
aqueous solution at room temperature, characterized by ICP-OES, XRD, TEM, SEM-EDS and XPS tech-
28 August 2017
Accepted 7 September 2017
Available online 8 September 2017
niques. The as-synthesized CuNi-CeO
employed as heterogeneous catalysts for the hydrolytic dehydrogenation of ammonia borane (NH
AB) under mild conditions. The optimal catalyst of Cu0.8Ni0.2-CeO /rGO with CeO content of 13.9 mol%
exhibited an excellent catalytic activity with the exceedingly high TOF value of
2
/rGO nanocomposites with various metal contents had been
3
BH ,
3
2
2
Keywords:
Ammonia borane
Dehydrogenation
Synergistic effect
Heterogeneous catalysis
ꢀ
1
ꢀ1
ꢀ1
,
3
4.4$molH2$molcatalyst$min at 298 K. The apparent activation energy was calculated to be 19.1 kJ mol
even lower than most of noble-metal-based catalysts for AB dehydrogenation. The detailed kinetics of AB
hydrolysis catalyzed by Cu0.8Ni0.2-CeO /rGO had been investigated on different concentrations of cata-
lyst, substrate and temperatures. Moreover, the obtained in situ hydrogen from AB hydrolysis can be
2
4-Nitrophenol reduction
ꢀ
1
further applied to reduce 4-nitrophenol with a high TOF value of 8.1 min at room temperature. The
high catalytic activity of Cu0.8Ni0.2-CeO /rGO would be contributed to the small size of CuNi-CeO NPs,
metal-metal synergy, and the metal-support interaction.
2
2
©
2017 Elsevier B.V. All rights reserved.
1
. Introduction
materials with a high specific surface area [11e15]. Reduced gra-
phene oxide (rGO) has been considered as an ideal support for
immobilization of metal NPs, owing to its fantastic electrical con-
ductivity, excellent thermal and chemical stability. The loading of
metal NPs on rGO can effectively restrain the aggregation of metal
NPs, as well as facilitate the electron transfer and mass transport
during the catalytic process [16,17].
On the other hand, hydrogen has been regarded as the prom-
ising candidate of clean energy in the future, owing to its high
energy density (142.2 MJ/kg) and only water as its product [18,19].
However, one of the major technological obstacles for the wide-
spread hydrogen on-board application is the effective storage of
Bimetallic nanoparticles supported on the suitable porous ma-
terials have attracted extensive attention in heterogeneous catal-
ysis, due to the synergistic effect of metal-metal and metal-support
interactions [1e3]. Bimetallic nanocatalysts often exhibit the great
enhancement on the catalytic activities than their component
counterparts, resulting from the intrinsic changes of the surface
geometric and electronic features [4e7]. In addition, the catalytic
performance of nanocomposites can be remarkable enhanced by
the electronic metal-support interactions, through the electronic
perturbation of the catalytic sites [8e10]. Therefore, it is fascinating
to exploit the heterogeneous catalysts of supported bimetallic
nanocomposites with high activity and selectivity. Considering that
catalytic reactions occur on the surface and interface, there are
cost-effective and fundamental incentives to design catalysts with
highly dispersed bimetallic NPs loading on the suitable support
3 3
hydrogen [20]. Ammonia borane (NH BH , AB) is the efficient car-
rier of hydrogen, owing to its high hydrogen content (19.6%), water
solubility and non-toxicity under ambient conditions [21,22]. Dur-
ing the hydrolytic dehydrogenation promoted by a suitable catalyst,
one equivalent of AB can generate three equivalents of hydrogen
gas according to Eq. (1) [23].
catalyst
þ
ꢀ
2
*
Corresponding author.
E-mail address: yhzhou@mail.ahnu.edu.cn (Y.-H. Zhou).
NH BH þ2H Oƒƒƒ!NH þBO þ 3H2
(1)
3
3
2
4
http://dx.doi.org/10.1016/j.jallcom.2017.09.075
925-8388/© 2017 Elsevier B.V. All rights reserved.
0

Y.-H. Zhou et al. / Journal of Alloys and Compounds 728 (2017) 902e909
903
Some representative nanocatalysts with different compositions
are listed in Table 1, in which the noble-metal-based catalysts
exhibit the high activities for the hydrolytic dehydrogenation of AB
nanocomposites (NCs) exhibited the significant enhancement on
the catalytic activity, accounting for the synergistic effect of metal-
metal and metal-support interaction. Moreover, kinetics studies on
the catalytic reduction of 4-nitrophenol with the reductant of the in
situ hydrogen from AB had been investigated in detail.
[6,9,20,22]. However, the practical applications are limited by their
low earth-abundance and high cost. Recently, first-row transition
metal NPs (such as Cu, Co and Ni) have been developed as the
economical alternatives for the catalytic dehydrogenation of AB
2. Experimental
[24,25]. Up to date, the catalytic performance on hydrogen gener-
ation could not meet the practical applications. Therefore, the
preparation of noble-metal-free catalysts with high activities for
the dehydrogenation of AB at mild conditions is highly desired.
Compared with monometallic counterparts, bimetallic nano-
composites exhibit much better activity for the dehydrogenation of
AB, due to the strong synergistic effect of metal-metal [1,7].
Furthermore, it had been reported that the catalytic dehydroge-
nation from AB over metal NPs could be greatly enhanced by
additional dopant of amorphous cerium oxide [26], with disturbing
the long-range order of metal NPs by decreasing the crystallinity
2.1. Materials
Sodium borohydride (NaBH
(CuCl $2H O), nickel(II) chloride hexahydrate (NiCl
cerium(II) nitrate hexahydrate (Ce(NO $6H O, 99.5%) were pur-
chased from Aladdin. Graphite powder, sulfuric acid (H SO , 98%),
phosphorous acid (H PO 85%), potassium permanganate
(KMnO ), hydrogen peroxide (H , 30%), hydrochloric acid (HCl,
4
, 98%), cupric chloride dihydrate
2
2
2
$6H O) and
2
3
)
3
2
2
4
3
4
,
4
2 2
O
30%), and borane ammonia complex (97%) were obtained from
Sigma Aldrich. All chemicals were used as obtained. The redistilled
water was used as the reaction solvent.
through strong metal-CeO
CeO NPs have a tendency of agglomeration after the catalytic
process. Therefore, the catalytic efficiency and stability would be
achieved by anchoring metal-CeO NPs on the support material of
2
interaction [16,27]. However, metal-
2
2.2. Characterization
2
rGO for the hydrolytic dehydrogenation of AB.
Furthermore, the in situ-generated hydrogen from AB can act as
a perfect hydrogen source to reduce nitro and olefin compounds
The crystal phase properties of the synthesized nanocatalysts
were analyzed with a Shimadzu X-ray diffractometer-6000 (XRD-
6000) using Cu Ka radiation at 40 kV and 40 mA (
l
¼ 0.1542 nm).
[28], overcoming the disadvantage of slow reaction rate for
The morphologies and sizes of the samples were determined by
using a transmission electron microscope (TEM, HT-7700), and
energy dispersive X-ray (EDX) spectroscopy on Field Emission
Scanning Electron Microscope (FESEM, S-4800) for elemental
analysis. Raman spectra were collected with a confocal Raman
microscope (LabRAM HR). X-ray photoelectron spectroscopy (XPS)
measurements were performed with a Thermo Fisher Scientific
ESCALAB 250X imaging electron spectrometer. Cu, Ni and Ce con-
tents of the catalyst samples were determined by using Agilent
7700ce inductively coupled plasma-optical emission spectroscopy
(ICP-OES) after sample was completely dissolved in a mixture of
hydrogen gas as reductant under high pressure and high temper-
ature [29]. The model substrate of 4-nitrophenol (4-NP) has been
extensively applied to evaluate the activities of catalysts in aqueous
solution [30,31]. Therefore, coupling of AB dehydrogenation and
reduction of 4-NP in tandem route is expected to achieve highly
efficiency.
Herein, we report the catalytic dehydrogenation from AB hy-
drolysis and cascade reduction of 4-NP over CuNi-CeO
ported on rGO as efficient catalysts at room temperature. For AB
hydrolysis, CuNi-CeO /rGO catalysts with various contents of
component had been optimized to be Cu0.8Ni0.2-CeO /rGO with
CeO content of 13.9 mol%, containing the exceedingly high TOF
value of 34.4$molH2$molcatalyst$min at 298 K. The apparent acti-
2
NPs sup-
2
2
3
HNO /HCl (1/3 ratio).
2
ꢀ1
ꢀ1
2.3. Preparation of catalyst
ꢀ1
vation energy was measured to be 19.1 kJ mol , even lower than
most of noble-metal-based catalysts for AB dehydrogenation. In
2
In a typical synthesis of Cu0.8Ni0.2-CeO /rGO, 20 mL aqueous
comparison with the component counterpart of Cu-CeO
2
/rGO, Ni-
/rGO
solution containing 10 mg of GO, which was synthesized by a
modified Hummers method [32], were ultrasonicated for 5 min to
CeO /rGO, CuNi NPs, CuNi-CeO and CuNi/rGO, CuNi-CeO
2
2
2
fully homogenize the contents. Then, 13.6 mg of CuCl
.8 mg of NiCl $6H O and 7.0 mg of Ce(NO $6H O were added to
the above suspension. The mixture was kept stirring for 1 h. After
that, 30 mg of NaBH was quickly poured into a reactor under rapid
2 2
$2H O,
4
2
2
3
)
3
2
Table 1
Catalytic activity of the nanocatalysts tested in the hydrolysis of ammonia borane.
4
stirring for 4 h under the nitrogen environment at room temper-
ature. The product was collected by centrifugation at 4000 rpm for
Catalysts
Temp (K)
n
metal/nAB
E
a
(kJ/mol)a TOF
Ref.
Cu0.2Co0.8/MCM-41
Co@Ni/rGO
298
298
298
298
298
298
298
303
303
298
298
298
298
298
0.05
0.05
0.0006
0.0079
0.05
0.0017
e
38
13.5
65
64.7
20.03
36.0
45.6
28
54.89
68
67.9
38.2
45
15.0 [3]
1
0 min, and copiously washed with water (3 ꢁ 10 mL), and dried in
16.4 [4]
ꢂ
ꢀ2
vacuum oven at 60 C under 10 Torrto give Cu0.8Ni0.2-CeO
(13.9 mol% Ce) as a black suspension. For synthesis of Cu
CeO
/rGO (x ¼ 1, 0.9, 0.8, 0.7, 0.5, 0.2, 0) with different compositions
were synthesized by using the above protocol but changing the
2
molar ratio of Cu: Ni. To debate the effect of CeO , Cu0.8Ni0.2-CeO /
2
/rGO
b
Pd/PDA-CoFe
Pt-CeO /rGO
2
O
4
175
[6]
b
x
Ni1-x-
2
93.8 [9]
102.4 [15]
320.7 [20]
303
8.8
Ag@Co/graphene
RuCo@MIL-96
2
Ru/TiO
2
(B)
[22]
[24]
2
Ni NPs/C
0.0425
0.05
0.011
0.04
0.1
rGO NCs (Ce/(Cu þ Ni þ Ce) ¼ 7.5, 13.9, 18.7 and 29 mol %) were also
CuCo/PDA-rGO
51.5 [25]
0
b
prepared by the above method by changing the initial amount of
Pd /CeO
2
29
24
[26]
[39]
b
Cu-Cu
Cu/rGO
Cu75Pd25/rGO
Ru@Al
2
O-CuO/C
3 3 2 2
Ce(NO ) $6H O. In addition, Cu0.8Ni0.2/rGO, Cu0.8Ni0.2-CeO , and
3.61 [43]
29.9 [45]
39.6 [46]
19.6 [47]
Cu0.8Ni0.2 samples were also synthesized by the same method.
b
e
2
O
3
e
48
27
2.4. Catalytic reactions
Nanoporous Ni spheres 298
e
Pd/rGO
Cu0.8Ni0.2-CeO
298
298
e
51
19.1
6.25 [48]
34.4 This work
2
/rGO
0.049
The activities of the catalysts were determined through
measuring the hydrogen generation rate during the hydrolysis of
AB for a specified amount of time in a typical water-filled gas
a
E
a
¼ the activation energy.
b
ꢀ1
ꢀ1
ꢀ1
TOF value is expressed as min , other is molH2 molcatalyst min
.

904
Y.-H. Zhou et al. / Journal of Alloys and Compounds 728 (2017) 902e909
Scheme 1. The synthetic rout of CuNi-CeO
2
supported on reduced graphene oxide.
burette system. Before the activity test, 10 mg catalyst was
dispersed in 5 mL of distilled water in a centrifuge tube via soni-
cation for 10 min. Next, the aqueous dispersion of catalyst was
transferred into the two-necked jacketed reaction flask (25 mL)
containing a Teflon-coated magnetic stir bar. And then, the flask
was placed on a magnetic stirrer and thermostated to 298 K. The
flask is well sealed with a silicon septum. To trigger the catalytic
of distilled water. Then 0.3 mL of catalyst (0.033 mM, calculated
form Cu and Ni, determined by ICP were quickly added. The in-
tensity of the absorption peak at 400 nm for 4-NP was monitored
by UVevis spectroscopy along with time.
3. Results and discussion
3 3
reaction, 1.0 mmol NH BH aqueous solution (5 mL) was rapidly
3
.1. Preparation and characterization of catalyst
injected into the solution of catalyst, and stirred at 800 rpm. The
volume of hydrogen gas generated was measured by recording the
displacement of water level at certain time intervals. The time was
periodically recorded with the water level interval of 2 mL in all
tests. The reaction was stopped when the water level was not
dropping.
CuNi-CeO
by wet-impregnation followed by simultaneous reduction
approach (Scheme 1). Under N atmosphere, Ce(NO was firstly
converted into Ce(OH) under the basic condition provided by the
hydrolysis of NaBH [16]. And then, amorphous CeO could be
formed by the redox reaction between Ce(OH) and metal pre-
cursors [33]. In addition, the GO can be also regarded as an oxidant
to oxidize Ce cations into CeO
rGO through GO reduction [9,34]. Consequently, the ultrafine CuNi-
CeO NPs were self-assembled by CuNi and CeO NPs, and then
subsequently grown on rGO nanosheets. The as-synthesized CuNi-
CeO /rGO was applied to be catalysts for the hydrolytic dehydro-
genation from NH BH in aqueous solution. Cu0.8Ni0.2-CeO /rGO
NCs were selected as the model catalyst for full characterization,
because it exhibited the highest catalytic performance among all
samples tested.
The as-synthesized catalysts were isolated by centrifugation
from the reaction solution, copiously washed with water, and
dehydrated in vacuum oven for characterization experiments. Fig. 1
2
/rGO catalyst was facilely and reproducibly prepared
2
3 3
)
3
4
2
3
2.5. Catalytic kinetics
3
þ
2
, accompanied with the forming
Three sets of experiment were carried out in the same way as
described in Section 2.4. In the first set of experiment, the hydro-
lysis reaction was performed at different catalyst amounts (5, 10, 15
2
2
and 20 mg Cu0.8Ni0.2-CeO
98 K. In the another set of experiments, the catalytic reaction was
performed at different NH BH concentration (50, 100, 200,
00 mM) by keeping the catalyst amount constant at 10 mg of
Cu0.8Ni0.2-CeO /rGO and temperature at 298 K. The final set of ex-
periments were performed at various temperatures in the range of
2 3 3
/rGO) by keeping 200 mM NH BH at
2
2
3
3
2
3
3
3
2
2
3 3
98e318 K by keeping the NH BH concentration constant at
2
00 mM and Cu0.8Ni0.2-CeO /rGO amount at 10 mg, in order to
2
a
obtain the catalytic activation energy (E ) based on the Arrhenius
equation.
depicts the powder XRD patterns of the as-synthesized samples. All
of them showed two crystalline peak at 2
ꢂ
q
¼ 43.3e44.5 and
2.6. Durability and reusability test
In a typical reusability test, 10 mg of catalyst was added into
mL aqueous solution of NH BH (200 mM) at 298 K for the first
5
3
3
run. After the first cycle of the hydrogen generation reaction was
completed, another equivalent of NH BH was added into the re-
3
3
action system and the gas released was monitored by using the gas
burette. The same process was continued five times.
3 3
2.7. Catalytic reduction of 4-nitrophenol by NH BH
The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP)
with NH BH as the reducing agent was investigated at ambient
3
3
temperature. The UVevis absorption spectroscopy was used to
evaluate the catalytic activity, as the reactant 4-NP anion has a
strong absorption peak at 400 nm, while the product 4-AP has a
medium absorption peak at 296 nm. Stock solutions of 4-NP
(
1
0.2 mM) and NH
.0 mL of 4-NP and 0.6 mL of NH
3
BH
3
(20 mM) were freshly prepared. Typically,
BH solution were mixed in 1.0 mL
Fig. 1. Powder XRD patterns of Cu0.8Ni0.2 NPs, Cu0.8Ni0.2/rGO, Cu0.8Ni0.2-CeO
Cu0.8Ni0.2-CeO /rGO.
2
NPs, and
3
3
2

Y.-H. Zhou et al. / Journal of Alloys and Compounds 728 (2017) 902e909
905
ꢂ
2
(
q
¼ 50.4e51.8 , which were attributed to face-centered cubic (fcc)
influence the growth and distribution of Cu0.8Ni0.2 NPs. However,
Cu0.8Ni0.2-CeO NPs prepared without GO were obviously aggre-
111) and (200) diffraction peaks. The bimetallic Cu0.8Ni0.2-CeO
2
/
2
rGO NCs presented diffraction peaks of (111) plane located at the
intermediary of monometallic Cu (JCPDS NO.04-0836) and Ni
gated to a bigger size (Fig. 2d). It would be attributed the fact that
the hydrophilic phenyl epoxide and hydroxyl groups, as well as the
hydrophobic basal plane of GO, could effectively anchor metal NPs
on the special surface and then control their distribution and
agglomeration on rGO nanosheets during the synthetic process
[37]. Furthermore, the energy-dispersive X-ray spectroscopy (EDX)
spectrum further confirms the existence of C, O, Cu, Ni and Ce el-
(
JCPDS NO.04-0850), indicating CuNi alloys were generated [35,36].
In the case of Cu0.8Ni0.2/rGO NCs, the weak characteristic peak at
ꢂ
2
q
¼ 36.4 were assigned to Cu
2
O [1]. In addition, the CeO
2
-doped
ꢂ
samples exhibited a broad and weak peak at around 2
corresponding to the amorphous state of CeO (JCPDS NO. 49-1415)
16]. It should be noted that these samples displayed a weaker XRD
q
¼ 26.3 ,
2
[
2
ements (Fig. S2). The contents of Cu, Ni and Ce in CuNi-CeO /rGO
peaks of CuNi alloy after doping of CeO
amorphous CeO as dopant can disturb the long-range order of
2
2
, due to the fact that
NCs were further determined by ICP-OES measurements (Table S1),
almost identical to their initial contents.
Cu0.8Ni0.2 alloy and thus decrease its crystallinity. In addition, it
could be found that the samples with a lower content of Cu
appeared a weaker crystalline peak of CuNi alloy from the powder
To investigate the compositions and chemical states of
Cu0.8Ni0.2-CeO /rGO NCs, the X-ray photoelectron spectroscopy
2
(XPS) were investigated (Fig. 3a). As shown in Fig. 3b, both signals
XRD patterns of CuNi-CeO
2
/rGO with different Cu/Ni molar ratio
of Ce(Ⅲ) and Ce(Ⅳ) appeared at Ce 3d level, attributed to the fact
(Fig. S1). Herein, it would be expected that the CuNi NPs with a low
2
that the oxygen defects in the structure of CeO can be dynamically
degree of crystallinity or in the amorphous state, featuring a short-
range order and long-range disorder, could improve the catalytic
performance of nanocatalyst.
and spontaneously transformed in response to the partial oxygen
pressure [9]. The XPS spectra of Cu 2p exhibited the major peaks at
binding energies (BEs) of 932.5 eV, corresponding to Cu(0). In
addition, the minor peak was located at 952.2 eV, accompanied by
low intensity satellites at 940e945 eV, could be respectively
assigned to Cu(I) and Cu(II) [38e40], suggesting that a thin oxidized
layer had been formed during exposure of the samples to air.
Meanwhile, from Fig. 3d, the Ni(0) 2p1/2 and 2p3/2 peaks appeared
at BEs 873.8 and 856 eV, respectively. Peaks with BEs 861.6 eV could
be attributed to Ni(II) 2p3/2 [13]. The XPS spectrum of C 1s in
As shown in Fig. 2, the sizes and morphologies of the as-
synthesized Cu0.8Ni0.2-CeO
2
/rGO, Cu0.8Ni0.2-CeO
2
and Cu0.8Ni0.2
/
rGO were characterized by TEM. As for Cu0.8Ni0.2-CeO
2
/rGO, all the
metallic NPs were highly dispersed on rGO nanosheets with an
average particle size of about 3.4 nm (Fig. 2a and b). In the case of
Cu0.8Ni0.2/rGO, CuNi nanoparticles exhibited the similar particle
2
size and dispersity (Fig. 2c), indicating that doped-CeO did not
2 2 2
Fig. 2. TEM images of(a) Cu0.8Ni0.2-CeO /rGO, (b) the size distribution for Cu0.8Ni0.2-CeO NPs in rGO (c) Cu0.8Ni0.2/rGO, and (d) Cu0.8Ni0.2-CeO NPs.

906
Y.-H. Zhou et al. / Journal of Alloys and Compounds 728 (2017) 902e909
(a)
(b)
Ce 3d5/2
Ce 3d3/2
Cu 2p
Ce 3d
O 1s
Ni 2p
C 1s
4
+
Ce
Ce
3
+
1
000
800
600
400
200
0
910
905 900 895 890
885
880
Binding Energy (eV)
Binding Energy (eV)
(
c)
Cu 2p3/2
(d)
Ni 2p3/2
Ni 2p1/2
Ni 2p3/2sat
Cu 2p1/2
9
55
950
945
940
935
930
875
870
865
860
855
Binding Energy (eV)
Binding Energy (eV)
2
Fig. 3. XPS spectra of (a) Cu0.8Ni0.2-CeO /rGO catalyst; (b) Ce 3d, (c) Cu2p, and (d) Ni 2p for the Catalyst.
Cu0.8Ni0.2-CeO
2
/rGO (Fig. S3) exhibited the signals of the oxygen
support and the doping-CeO
(Fig. S6), CuNi NPs with CeO
behave much better catalytic activity. Cu0.8Ni0.2-CeO
Cu0.8Ni0.2 NPs with the doping of amorphous CeO supported on
2
was inactive for the catalytic reaction
dopant and rGO support would
/rGO,
containing functional groups (such as -COOH, -C¼O, -C-O). And
their peak intensities were weaker than those in GO, verified the
reduction of GO [41]. It could be further determined from Raman
2
2
2
spectra (Fig. S4). Both the as-synthesized GO and Cu0.8Ni0.2-CeO
rGO exhibit two peaks centered at 1347 and 1600 cm , assigned to
the D and G bands of carbon products, respectively. The intensity
2
/
rGO nanosheets, exhibited a greatly enhancement on catalytic
performance towards hydrogen generation from AB released in
only 1.8 min (Fig. 5). Furthermore, its turnover frequency (TOF)
based on CuNi atoms was calculated to be 34.4
ꢀ1
ratio of D and G (I
the graphene-based materials [42]. The I
from 0.74 to 1.16 after the loading of Cu0.8Ni0.2-CeO
relative enhancement on the value of I /I was accounted for the
D
/I
G
) bands could be helpful to analyze defects of
/I value of GO altered
NPs. The
ꢀ1
ꢀ1
D
G
molH2$molcatalyst$min . As listed in Table 1, the TOF value of
Cu0.8Ni0.2-CeO /rGO for AB hydrolysis was relatively higher than
2
2
D
G
most of the reported nanocatalysts [45e48], suggesting that this
nanocomposite was an efficient catalyst for the hydrolysis of AB.
Moreover, to investigate the kinetics properties of the hydrolysis
GO reduction during the in situ fabrication [43,44], which was in
accordance with the XPS analyses.
dehydrogenation from AB catalyzed by Cu0.8Ni0.2-CeO
2
/rGO, the
reaction rates were studied at different concentrations of catalysts
3.2. Catalytic dehydrogenation of ammonia borane
x 2
The catalytic performance of the as-synthesized Cu Ni1-x-CeO /
rGO (x ¼ 1, 0.9, 0.8, 0.7, 0.5, 0.2, 0) NCs was assessed by the hy-
drolysis of AB in aqueous solution. Reaction was initiated by adding
AB into the aqueous solution containing catalyst with vigorous
agitation at 298 K. As shown in Fig. 4, monometallic Cu-CeO
2
/rGO
exhibited certain activity, whereas monometallic Ni-CeO /rGO had
2
almost no activity towards the hydrolysis of AB within 20 min at
room temperature. It was impressive that all the bimetallic CuNi-
CeO
2
/rGO catalysts had much better activities than both mono-
/rGO and Ni-CeO /rGO. The catalytic rate
metallic Cu-CeO
2
2
enhanced at first and then decreased in the range of Cu molar
content from zero to 100%, in which the optimal nanocomposite of
Cu0.8Ni0.2-CeO
the complete yield of hydrogen products. The enhancement on the
catalytic performance of CuNi-CeO /rGO NCs could be contributed
2
/rGO exhibited the highest catalytic activity with
2
to the synergistic effect between Cu and Ni induced by the so-called
strain and ligand effects [7,27]. In addition, when the molar content
of Ce is optimized to be 13.9 mol%, the resulting Cu0.8Ni0.2-CeO
2
/
2
Fig. 4. Volume of the generated H versus time for the hydrolytic dehydrogenation of
rGO has the best catalytic performance (Fig. S5). Since only rGO
AB over CuNi-CeO /rGO with different molar ratio of Cu/Ni at 298 K.
2

Y.-H. Zhou et al. / Journal of Alloys and Compounds 728 (2017) 902e909
907
increased with the elevated temperature. The E
a
value for AB
ꢀ
1
dehydrogenation could be calculated to be 19.1 kJ mol
Arrhenius plot (ln k vs 1/T), depicted in the inset of Fig. 6. As listed in
Table 1, the E value of Cu0.8Ni0.2-CeO /rGO catalyst for AB hydro-
from
a
2
lysis was relatively lower than that of the reported noble-metal-
free nanocatalysts, even outperform some noble-metal nano-
composites [46e49]. Therefore, Cu0.8Ni0.2-CeO /rGO nano-
2
composite could be a promising catalyst for the hydrolytic
dehydrogenation of AB.
The durability and recyclability of catalyst were important to the
practical application. The durability and reusability of Cu0.8Ni0.2
CeO /rGO for hydrogen generation from AB in aqueous solution up
to fifth run were evaluated at 298 K. As shown in Fig. 7, no signif-
icant decay in the activity of Cu0.8Ni0.2-CeO /rGO was observed
-
2
2
after the five successive runs. The stability and durability of the
heterocatalyst could be confirmed by the TEM images. As shown in
Fig. S8, the size and morphology of metal NP on rGO had no
noticeable change even after fifth runs in the reusability test,
indicating the good durability in the catalytic dehydrogenation
from AB [50].
2
Fig. 5. Volume of the generated H versus time for the hydrolytic dehydrogenation of
AB catalyzed by different nanocomposites at 298 K.
and AB [49]. Fig. S7a showed the plots of H
the hydrolysis of AB (200 mM) at different Cu0.8Ni0.2-CeO
2
volume vs time during
/rGO
concentrations (5e20 mg) at 298 K. The hydrogen generation rate
v) was determined from the linear portion of the plot for each
2
3.3. Chemical reduction of 4-nitrophenol by ammonia borane
(
The reduction of nitro compound to amines in a facile and well-
controlled way plays a vital role on organic synthetic chemistry,
and 4-nitrophenol (4-NP) was usually selected as a model substrate
catalyst concentration. As depicted in Fig. S7b, the catalytic rate
exhibited a linear growth with catalyst concentration. The slope of
the fitting straight line in the plot of ln rate vs ln [catalyst] was
found to be 0.97, which was very close to 1, indicating that the
hydrolysis of AB over Cu0.8Ni0.2-CeO /rGO was a first-order reaction
2
with respect to the amount of the catalyst [45]. Whereas, a fitting
straight line with a slope of 0.18 was obtained (Fig. S7c), demon-
2
strating that the AB hydrolysis catalyzed by Cu0.8Ni0.2-CeO /rGO
was an approximately zero-order reaction with respect to the
amount of substrate. Therefore, the rate formula for AB hydrolysis
[
51]. As a mild hydrogen source during the process of catalytic
hydrogenation, ammonia borane can overcome the instability and
toxicity of the reductants of NaBH [52]. It is well known that the
4
reduction process of 4-NP can be monitored by measuring the
UVeVis absorption spectra of the reaction solution. A continuous
decay of the absorption peak at 400 nm is directly associated with
the reduced concentration of 4-nitrophenolate ions [53]. As shown
in Fig. 8a, the absorption peak of the mixture of 4-NP and AB is at
catalyzed by Cu0.8Ni0.2-CeO
Catalyst], in which k was the rate constant.
Furthermore, in order to determine the activation energy (E
AB hydrolysis catalyzed by Cu0.8Ni0.2-CeO /rGO, the reaction rate
2
/rGO would be expressed as v ¼ k
3
17 nm, indicating strong stability of AB in water in the absence of
[
catalyst. However, after addition of Cu0.8Ni0.2-CeO /rGO, the ab-
2
a
) of
sorption intensity at 400 nm immediately increased, while the peak
at 317 nm rapidly decreased at the same time. This phenomenon is
2
had been measured at different temperatures [7]. Fig. 6 showed the
plots of hydrogen volume versus reaction time during AB hydrolysis
at different temperatures with the range from 293 K to 308 K. The k
value of AB hydrolysis was estimated from linear portion. The re-
þ
attributed to the formation of 4-NP anions through H ions disso-
ciation from 4-NP in a basic solution provided by the hydrolysis of
AB [31]. When the reaction proceeds to 60 s, the absorption peak at
00 nm reaches the maximum value, and then gradually decreases
4
2
action rate from AB hydrolysis catalyzed by Cu0.8Ni0.2-CeO /rGO
from 60 to 210 s until the complete degradation, accompanied with
Fig. 6. Volume of the generated H
AB catalyzed by Cu0.8Ni0.2-CeO
/rGO at different temperatures (T ¼ 298e318 K), and
inset) Arrhenius plot.
2
versus time for the hydrolytic dehydrogenation of
2
Fig. 7. Durability test of catalytic hydrolysis of AB over Cu0.8Ni0.2-CeO rGO at 298 K.
Additional aliquots of AB were subsequently added into the reaction vessel after the
completion of last runs.
2
(

908
Y.-H. Zhou et al. / Journal of Alloys and Compounds 728 (2017) 902e909
0 2
Fig. 8. (a) Successive UVevis absorption spectra showing the reduction of 4-NP by AB. (b) Plot of time vs ln(C/C ) by Cu0.8Ni0.2-CeO /rGO at different catalyst concentrations. (c) Plot
of time vs ln(C/C ) by Cu0.8Ni0.2-CeO /rGO at different AB concentrations. (d) Relationship between ln k and AB or catalyst concentration.
0
2
the appearance of a new peak at 296 nm assigned to the product of
-aminophenol (4-AP). Meanwhile, the bright yellow solution
completely faded and became colorless. The rate constant k can be
calculated from the equation ln(C/C represent the
) ¼ kt (C and C
concentration of 4-NP at time t and at the beginning of the reaction,
respectively). Fig. 8b depicted the plot of ln(C/C ) vs t with different
concentrations of catalyst at room temperature. A rapid and almost
linear evolution is observed between ln(C/C ) and t, indicating that
hydrogen generation from AB with a relatively higher TOF value,
attributed to the small metal NPs and synergistic effect among the
components of catalyst. In addition, the in situ hydrogen was
applied to further reduce 4-NP to 4-AP. The kinetics studies
revealed that the catalytic reduction of 4-NP was first-order with
respect to 4-NP concentration and zero-order with respect to AB
concentration. Such non-noble-metal nanoparticles supported on
rGO would have a potential application prospect in many catalytic
reactions.
4
0
0
0
0
the catalytic reaction is a typical first-order reaction with respect to
the concentration of 4-NP. Increasing the catalyst concentration
dramatically increases the catalytic rate. The plot of ln(C/C
0
) vs time
Acknowledgements
at various AB concentrations is also shown in Fig. 8c. However, it
was wondrously found that the k value remaining almost constant,
and the increasing of the dosage of AB did not increase the reaction
rate of catalyst [16]. The relationship between k and the concen-
tration of AB or the catalyst is illustrated in Fig. 8d. From the nearly
horizontal line about the k change with AB amount, it was found
that this was a zero-order reaction with respect to AB concentra-
tion. Furthermore, to determine the efficiencies of catalyst, the
turnover frequency (TOF) was calculated to be 8.10 mol4-AP
This work was supported by the National Natural Science
Foundation of China (Grant NO. 21771004), Natural Science Foun-
dation of Anhui Province (1408085MKL21), Special and Excellent
Research Fund of Anhui Normal University, and Undergraduate
Innovative training program of Anhui Normal University (Grant No.
201610370486, 201610370490).
Appendix A. Supplementary data
ꢀ
1
ꢀ1
2
molcatalyst min . As shown in Table S2, Cu0.8Ni0.2-CeO /rGO
exhibited the relatively higher catalytic efficiencies than most of
other catalysts for the reduction of 4-NP [16,30,31]. It may be
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jallcom.2017.09.075.
attributed to the small size of CuNi-CeO
and the metal-support interaction.
2
NPs, metal-metal synergy,
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