Fabrication of a Ti-supported NiCo2O4 nanosheet array and its superior catalytic performance in the hydrolysis of ammonia borane for hydrogen generation

Article abstract of DOI:10.1039/c5cy01542a

The development of low-cost nanocatalysts with high activity, high stability and good reusability towards the hydrolysis of chemical hydrides for hydrogen generation is of great importance and significance in the field of hydrogen energy. In this work, a Ti-supported NiCo₂O₄ thin film nanosheet array (NiCo₂O₄/Ti) is fabricated by a facile method, and its catalytic performance in the hydrolysis of ammonia borane for hydrogen production is investigated. It is found that the turnover frequency (TOF) of NiCo₂O₄/Ti can reach 50.1 mol H₂ min^-1 (mol catalyst)^-1, which is the highest TOF value reported for noble-metal-free catalysts towards the hydrolysis of ammonia borane. The apparent activation energy of ammonia borane hydrolysis in the presence of the NiCo₂O₄/Ti catalyst is as low as ca. 17.5 kJ mol^-1. More importantly, the NiCo₂O₄/Ti catalyst can retain ca. 90% of its original catalytic activity after 10 cycles, exhibiting much improved durability and reusability in contrast to many nanocatalysts recently reported in the literature. Its high catalytic activity and low-cost, together with its good durability and reusability, enable NiCo₂O₄/Ti to be a strong catalyst candidate for the hydrolysis of ammonia borane for hydrogen production in the practical applications.

Full text of DOI:10.1039/c5cy01542a

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Fabrication of a Ti-supported NiCo2O4 nanosheet
array and its superior catalytic performance in the
hydrolysis of ammonia borane for hydrogen
generation†
Cite this: DOI: 10.1039/c5cy01542a
*
Jinyun Liao, Hao Li, Xibin Zhang, Kejun Feng and Yanling Yao
The development of low-cost nanocatalysts with high activity, high stability and good reusability towards
the hydrolysis of chemical hydrides for hydrogen generation is of great importance and significance in the
field of hydrogen energy. In this work, a Ti-supported NiCo2O4 thin film nanosheet array (NiCo2O4/Ti) is
fabricated by a facile method, and its catalytic performance in the hydrolysis of ammonia borane for hydro-
gen production is investigated. It is found that the turnover frequency (TOF) of NiCo2O4/Ti can reach 50.1
mol H2 min−1 (mol catalyst)−1, which is the highest TOF value reported for noble-metal-free catalysts to-
wards the hydrolysis of ammonia borane. The apparent activation energy of ammonia borane hydrolysis in
the presence of the NiCo2O4/Ti catalyst is as low as ca. 17.5 kJ mol−1. More importantly, the NiCo2O4/Ti
catalyst can retain ca. 90% of its original catalytic activity after 10 cycles, exhibiting much improved durabil-
ity and reusability in contrast to many nanocatalysts recently reported in the literature. Its high catalytic
activity and low-cost, together with its good durability and reusability, enable NiCo2O4/Ti to be a strong
catalyst candidate for the hydrolysis of ammonia borane for hydrogen production in the practical
applications.
Received 12th September 2015,
Accepted 25th December 2015
DOI: 10.1039/c5cy01542a
www.rsc.org/catalysis
with two moles of water in the presence of an appropriate cat-
alyst via the following reaction equation:
Introduction
The consumption of energy in the world keeps on rising and
the present energy system based on fossil fuels produces a
large amount of pollutants, including carbon dioxide, which
is regarded as a major cause of the “greenhouse effect”. There
is no doubt that seeking sustainable, environment-friendly
and cost-effective energy sources is one of the most important
tasks in the 21st century. Hydrogen is considered to be an
ideal fuel for heating, mechanical power, transportation, as
well as electricity generation, on account of its high energy
density (142 MJ kg−1), zero emissions and sufficient source.1,2
Over the past few decades, a number of techniques have been
developed to generate and store hydrogen. Particularly, the
production of hydrogen by catalytic hydrolysis of chemical hy-
drides, such as NaBH4, N2H4 and NH3BH3 (ammonia borane,
AB), has received much attention in view of its high effi-
ciency, convenience and safety.3–6 For example, three moles
of hydrogen will be produced when one mole of AB reacts
+
−
H3NBH3(aq) + 2H2O(l) → NH4 (aq) + BO2 (aq) + 3H2(g) (1)
In the above hydrolytic reaction, the catalyst plays a cru-
cial role in determining the efficiency and rate of hydrogen
generation. Over the past decade, it has been well demon-
strated that noble-metal-based catalysts, such as Pt,5,7 Ru,8–10
Pd,11 CoPd,12 CuPd,13 NiPd (ref. 14) and PdPt,15 exhibit rela-
tively high catalytic activity in AB hydrolysis. However, their
large-scale applications were restricted by high costs and
scarcity. In contrast, noble-metal-free catalysts are more at-
tractive due to their greater potential in practical applica-
tions. In recent years, many efforts have been devoted to the
development of low-cost catalysts for AB hydrolysis and nu-
merous inexpensive nanocatalysts have been reported, in-
cluding Co,16–18 Ni,19 CoB,20 CoP,21 and Co–Mo–B–P.22 How-
ever, most of them suffer from difficult isolation from the
reaction solutions, lower activity in contrast to noble metal
catalysts, and poor stability or reusability. So, the develop-
ment of cost-effective, highly active and stable catalysts for
AB hydrolysis is highly desirable, which is still a big chal-
lenge. On the other hand, most of these low-cost nano-
catalysts for AB hydrolysis reported in the literature are Co,
Department of Chemical Engineering, Huizhou University, Huizhou, 516007,
China. E-mail: lihao180@126.com; Fax: +86 752 2527229; Tel: +86 752 2527229
† Electronic supplementary information (ESI) available: Digital camera photos
and SEM images. See DOI: 10.1039/c5cy01542a
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Ni or their alloys. Therefore, it is of great importance and sig-
nificance to develop other types of catalysts rather than
metals or alloys.
sodium citrate at a molar ratio of 2 : 1 : 6 were mixed into
water under intense stirring to form a mixed solution. After
the addition of the NaOH solution, the mixed solution was
subjected to a hydrothermal treatment at 160 °C for 10 h.
Then, the collected black powder was heated at 400 °C for 4 h.
In this work, a Ti-supported NiCo2O4 nanosheet array was
fabricated by a facile process and its catalytic performance in
the hydrolysis of AB was investigated. It should be mentioned
that the NiCo2O4 nanostructures for high-performance super-
capacitors have been intensively reported over the past few
years;23–25 however, their applications as catalysts in hetero-
geneous catalysis have been seldom dealt with. As far as we
know, this is the first investigation on NiCo2O4 nanostruc-
tures as a catalyst towards AB hydrolysis for hydrogen pro-
duction. On the other hand, considering that nanostructured
catalysts in the form of film will display better durability and
reusability, the Ti-supported NiCo2O4 nanosheet array rather
than NiCo2O4 nanostructures in the form of powder is
designed in the present study. Considering its low cost, high
mechanical strength, together with its high stability in differ-
ent chemical environments, Ti is selected as the substrate in
our work. It is interesting to find that the turnover frequency
(TOF) of NiCo2O4/Ti in AB hydrolysis can reach 50.1 mol H2
min−1 (mol catalyst)−1. To the best of our knowledge, this is
the highest TOF value reported in the literature for noble-
metal-free catalysts towards AB hydrolysis. In addition, the
Ti-supported NiCo2O4 nanosheet array exhibits significantly
improved durability and reusability compared with those
nanocatalysts recently reported in the literature, which en-
able it to be a strong catalyst candidate in AB hydrolysis for
the generation of hydrogen for practical applications.
Characterization
X-ray diffraction (XRD) patterns were recorded using a
PANalytical B. V. Empyean X-ray diffractometer with Cu Kα
radiation (λ = 1.5406 Å). The morphology of the array was in-
vestigated using a Hitachi S-4800 field emission scanning
electron microscope (FE-SEM). TEM images were obtained on
a Tecnai G2 F20 S-TWINT transmission electron microscope.
X-ray photoelectron spectra (XPS) were obtained on a VG
ESCALAB 210 X-ray photoelectron spectrometer with Al Kα ra-
diation. The specific surface area was measured on
a
Quantachrome Autosorb-1 volumertic analyzer, using nitro-
gen adsorption and the Brunauer–Emmett–Teller method.
The weight of NiCo2O4 loaded on the Ti substrate was mea-
sured by a Varian 720 Inductively Coupled Plasma-Optical
Emission Spectrometer (ICP-OES).
Catalytic performance testing
The catalytic performance of the NiCo2O4/Ti and NiCo2O4
nanoparticles were assessed by measuring the accumulative
volume of hydrogen generated during the AB hydrolysis reac-
tion in a glass reactor connected with a gas burette. Typically,
20.0 mL of freshly-prepared mixed solution of AB (0.15 M)
and NaOH (1 M) was added in the reactor. Then, the
NiCo2O4/Ti (5 cm × 3 cm) was added into the AB solution.
The reaction temperature was maintained at 308 0.5 K in a
thermostated reactor. The reusability of the NiCo2O4/Ti was
evaluated by repeating the hydrolysis reaction 10 times in
such a model: (1) recollecting the catalyst after the reaction;
(2) washing the catalyst with water and ethanol alternately
before drying at 50 °C in a vacuum oven; and (3) starting a
new run of AB hydrolysis by mixing the catalyst with freshly-
prepared AB solution.
Experimental section
Synthesis
All reagents were of analytical grade and used as received
without further purification. Double-distilled water was
used throughout the experiments. To prepare the NiCo2O4
nanosheet array supported on Ti substrate, 4.0 mmol of
CoCl2·6H2O and 2.0 mmol NiCl2·6H2O was mixed into 20.0
mL of double-distilled water under stirring to form solution
A, and 12.0 mmol sodium citrate was dissolved into 20.0 mL
of water to form solution B. After solution A and B were
blended, 40.0 mL of NaOH (7.5 M) solution were dropped
into the above solution. Afterwards, the mixed solution was
quickly transferred into a Teflon-lined steel autoclave, in
which a piece of 5 × 12 cm Ti sheet with a thickness of ca. 20
μm and a weight of ca. 0.72 g (purchased from Qingyuan Me-
tallic materials Co., Ltd., China) closely attached to the inner
wall of the vessel was used as support. Finally, the autoclave
was kept in an air oven at 160 °C for 10 h. After that, the
sample was transferred into a muffle furnace and subjected
to a heat treatment at 400 °C for 4 h. After the reaction, the
film was taken out and washed with double-distilled water
and acetone, then dried in a vacuum oven at 60 °C for 5 h.
For comparison, the NiCo2O4 nanoparticles were also pre-
pared by a similar process except that no Ti substrate was
used in the synthesis. Briefly, CoCl2·6H2O, NiCl2·6H2O and
Results and discussion
The digital camera photos of the bare Ti sheet and NiCo2O4/Ti
are presented in Fig. S1 (see the ESI†). As can be seen, the
bare Ti substrate is silver gray with metallic luster (Fig. S1a†)
while NiCo2O4/Ti is black (Fig. S1b†), implying that the black
NiCo2O4 has been successfully deposited on the Ti sheet in
this study. The microstructures on the surface of the Ti sub-
strate and NiCo2O4/Ti are investigated by FE-SEM. As can be
observed in Fig. 1, the Ti substrate is very smooth on its sur-
face (Fig. 1a). In contrast, it became very coarse after the de-
position of NiCo2O4 (Fig. 1b). Fig. 1c reveals that the NiCo2O4
layer on the substrate is comprised of the NiCo2O4 nanosheet
array, which looks somewhat like flowers. Fig. 1d indicated
that the mean thickness of the nanosheets is about 20 nm.
Obviously, these nanosheets are not loosely agglomerated but
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Fig. 2 XRD patterns of the bare Ti substrate (a), the as-prepared
NiCo2O4/Ti (b) and the NiCo2O4/Ti after 10 catalytic cycles (c).
NiCo2O4 (PDF#73-1702). This result further confirms that the
NiCo2O4 has been successfully deposited on the Ti substrate.
To obtain the information about the chemical states of Co
and Ni in NiCo2O4/Ti, XPS analysis was carried out. As
Fig. 1 SEM images of the bare Ti substrate (a) and NiCo2O4/Ti (b–d).
TEM image of the NiCo2O4 nanosheet array (e) and cross-section TEM
image of a piece of NiCo2O4 nanosheet (f).
closely interconnected to each other, which is marked with
the dotted circles in Fig. 1d. It is worth noting that an ultra-
sonic treatment of 30 min cannot detach these NiCo2O4
nanosheets from the Ti substrate, hinting that the as-
prepared NiCo2O4 nanosheet array is very stable. Besides, a
peeling test using scotch tape was also carried out (experi-
ment details about the peeling test can be seen in the ESI†),
and it was found that no NiCo2O4 was detached from the sub-
strate after the test. This further demonstrates the good sta-
bility of our sample. Fig. 1e shows the TEM image of the
NiCo2O4 nanosheet array, which further confirms the
nanostructures of the NiCo2O4 nanosheets. Fig. 1f shows the
cross-section TEM image of a piece of NiCo2O4 nanosheet,
demonstrating a thickness of ca. 20 nm. The ICP-OES analy-
sis indicates that the weight of NiCo2O4 loaded on the Ti
sheet (5 × 12 cm) is ca. 9.6 mg.
The crystal structures of the Ti sheet-supported NiCo2O4,
as well as the Ti substrate, were investigated by XRD. As
depicted in Fig. 2, seven characteristic peaks at 2θ = 35.2°,
38.4°, 40.3°, 53.0°, 63.0°, 70.6° and 76.2° are observed in the
XRD pattern of the Ti substrate, which can be indexed to the
(100), (002), (101), (102), (110), (103) and (112) plane reflec-
tions of the hexagonal close-packed phase of Ti (PDF#65-
9622). Besides these seven peaks, eight characteristic peaks
appear in the XRD pattern of NiCo2O4/Ti, which can be
assigned to the diffractions from (220), (311), (400), (331),
(442), (511), (440) and (533) planes of the cubic phase of
Fig. 3 XPS spectra of NiCo2O4/Ti in the regions of Ni 2p (a) and Co
2p (b).
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displayed in Fig. 3, the deconvoluted XPS spectrum shows
two peaks at the binding energies of ca. 872.8 and 871.3 eV
in the region of Ni 2p1/2, and two peaks at the binding ener-
gies of ca. 855.2 and 853.7 eV in the region of Ni 2p3/2. The
peaks at the binding energies of ca. 872.8 and 855.2 eV can
be indexed to NiIJIII), while those at the binding energies of
ca. 871.3 and 853.7 can be assigned to NiIJII).26 Similarly, the
deconvoluted XPS spectrum shows two peaks at the binding
energies of ca. 796.0 and 794.7 eV in the region of Co 2p1/2
,
and two peaks at the binding energies of ca. 780.7 and 779.6
eV in the region of Co 2p3/2. The peaks at the binding ener-
gies of ca. 779.6 eV and 794.6 eV can be indexed to CoIJIII),
while those at the binding energies of ca. 780.7 eV and 796.0
eV can be assigned to CoIJII).26 XPS analysis reveals that Co2+,
Co3+, Ni2+ and Ni3+ coexist on the surface of the NiCo2O4/Ti
sample, which is similar to the XPS results of the NiCo2O4
nanostructures reported by other groups.24,26,27
The release of hydrogen from the AB solution catalyzed by
the as-prepared NiCo2O4/Ti was investigated. First, the cata-
lytic activity of the Ti substrate in the AB hydrolysis is
checked. It was found that nearly no hydrogen is produced
from the AB solution when the bare Ti sheet is presented in
the reaction system, demonstrating that the Ti substrate is
inactive towards AB hydrolysis. When NiCo2O4/Ti is added
into the AB solution, a certain amount of hydrogen will be
generated, suggesting that the NiCo2O4 on the Ti substrate
can catalyze the AB hydrolysis. Fig. 4a shows the accumulated
volume of hydrogen vs. the hydrolysis time at catalyst dos-
ages ranging from 0.3 to 3.6 mg. It is observed that the accu-
mulated hydrogen volume is almost directly proportional to
the reaction time at the early stage of the hydrolysis, hinting
that the hydrolysis process follows the zero-order kinetics
with respect to ammonia borane. To confirm this, a series of
catalytic hydrolysis experiments with different initial concen-
trations of AB were carried out and the results are shown in
Fig. S2 in the ESI.† As can be seen in Fig. S2,† the rate con-
stants almost remain the same despite the fact that the ini-
tial concentrations of AB in the catalytic process are different,
demonstrating that the hydrolysis process follows the zero-
order kinetics with respect to AB. Note that a deviation from
the linear dependence of the accumulated hydrogen volume
with the reaction time was observed at the late stage of the
hydrolysis reaction, which may result from the external diffu-
sion limitation at a low concentration of AB.7 The rate con-
stant of AB hydrolysis can be determined from the linear por-
tion of each plot, which can be formulated as follows:
Fig.
4
(a) The accumulated volume of generated hydrogen vs.
reaction time at different catalyst dosages; (b) the rate constants at
different catalyst dosages.
hydrolysis of AB catalyzed by NiCo2O4 and the kinetics data
on it is unavailable in the literature. For comparison,
NiCo2O4 particles with a typical diameter of 15–25 nm were
prepared and the SEM images are shown in Fig. S3 (see the
ESI†). A series of contrast experiments were carried out, in
which the NiCo2O4 nanoparticles instead of NiCo2O4/Ti acted
as catalysts. The rate constants of AB hydrolysis in the pres-
ence of the NiCo2O4 nanoparticles and the NiCo2O4/Ti as cat-
alysts were compared and the results are indicated in
Table 1. It is interesting to note that the rate constants of AB
hydrolysis for the NiCo2O4/Ti catalyst are 10–12 times as large
as those for the NiCo2O4 nanoparticles although the BET sur-
face area of NiCo2O4/Ti is only ca. 1.4 times as large as that
of the NiCo2O4 nanoparticles. Two factors account for the fact
that the NiCo2O4/Ti catalyst exhibits better catalytic perfor-
mance in contrast to the NiCo2O4 nanoparticles. First, the
NiCo2O4 layer in the NiCo2O4/Ti catalysts are composed of
nanosheets, which are expected to possess more corners and
edges compared with the spherical NiCo2O4 nanoparticles. In
general, the atoms on the corners or edges have unsaturated
valency with lower number of bonds around them compared
with those in the interiors or on the faces.28,29 Thus, such
atoms will display much higher intrinsic catalytic activity.
k = V/t
where k, V, t are the rate constant, accumulated volume of hy-
drogen and hydrolysis time, respectively. Fig. 4b depicts the
effect of catalyst dosage on the rate constants. Clearly, k in-
creases linearly with the mass of NiCo2O4. This implies that
the hydrogen production rate can be easily controlled by
adjusting the amount of NiCo2O4 catalysts. It should be men-
tioned that this is the first investigation into the catalytic
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Table 1 The BET surface of the NiCo2O4/Ti and NiCo2O4 nanoparticles, and the comparison of rate constants of AB hydrolysis catalyzed by NiCo2O4/Ti
and NiCo2O4 nanoparticles at different dosages
Rate constant (min−1
)
BET surface
Sample
0.3 mg
0.6 mg
1.2 mg
2.4 mg
3.6 mg
(m2 g−1
)
NiCo2O4/Ti
NiCo2O4 nanoparticles
5.4
0.51
8.2
0.77
11.2
1.03
15.8
1.36
19.5
1.61
65.5a
47.2
a
The mass in the BET surface of NiCo2O4/Ti only counts the mass of NiCo2O4, not including that of Ti substrate.
Second, the NiCo2O4 nanoparticles suspended in the AB solu-
tion tend to aggregate on account of the high surface energy.
Under such circumstances, the number of active sites of
particulate NiCo2O4 will drop, which will result in the de-
crease of their catalytic activity. On the contrary, the NiCo2O4
nanosheets in the NiCo2O4/Ti catalyst are immobilized on the
Ti sheet. In this case, the decline of catalytic performance
which resulted from the aggregation is inevitable.
In this study, AB hydrolysis catalyzed by the NiCo2O4/Ti
catalyst was carried out at different reaction temperatures
and the relationship between the volumes of hydrogen and
the reaction time is displayed in Fig. 5a. It is found that ele-
vating the reaction temperature from 298 K to 308 K will
cause a pronounced increase in the hydrolysis rate of AB.
While a further elevation of temperature to 318 K and 328 K
only results in a slight increase of hydrolysis rate of AB. The
corresponding TOF value of the NiCo2O4/Ti sample at 298 K
is figured out to be ca. 50.1 mol H2 min−1 (mol catalyst)−1.
For comparison, the TOF values of some noble-metal-based
and noble-metal-free nanocatalysts recently reported in the
literature are also listed in Table 2. Evidently, our NiCo2O4/Ti
catalyst falls into the class of high-performance catalysts to-
wards AB hydrolysis. To the best of our knowledge, among
the noble-metal-free heterogeneous catalysts for AB hydroly-
sis, the NiCo2O4/Ti catalyst exhibits the highest catalytic activ-
ity in terms of TOF.
The apparent activation energy (Ea) is an important pa-
rameter for a catalytic reaction, which can be utilized to as-
sess the overall catalytic activity of a catalyst. Generally, a cat-
alyst with higher catalytic activity always leads to lower
activation energy of the catalytic reaction. According to the
Arrhenius formula, the apparent activation energy of AB hy-
drolysis catalyzed by the NiCo2O4/Ti catalyst can be figured
out based on the data on rate constants at different tempera-
tures. Fig. 5b displays the logarithm of the rate constants vs.
the reciprocal of the corresponding reaction temperatures.
According to the slope of the fitted line in Fig. 5b, the Ea
value for AB hydrolysis catalyzed by the NiCo2O4/Ti catalyst
was calculated to be 17.5 kJ mol−1. This value is even much
lower than the Ea value for AB hydrolysis catalyzed by noble-
metal-based nanocatalysts (Table 2), hinting at the superior
catalytic activity of the NiCo2O4/Ti catalyst.
In addition to the activity, the recyclability of a heteroge-
neous catalyst is of great importance in practical applica-
tions. In the present study, the hydrolysis of AB catalyzed by
the NiCo2O4/Ti catalyst was repeated 10 times. The relation-
ship between the accumulated volume of generated hydrogen
and reaction time at different cycles is shown in Fig. S4.† The
normalized rate constant, viz. kn/k1, where k1 and kn severally
denotes the rate constant of AB hydrolysis in cycles No. 1 and
No. n, is utilized to assess the durability and reusability of
the NiCo2O4/Ti catalyst. As displayed in Fig. 6, the hydrolysis
efficiency decreases marginally from 96% at the first cycle to
92% at the 10th cycle. As for the rate constant of AB hydroly-
sis, it almost remains unchanged in the first 6 cycles and de-
creases slightly in the following cycles. At the 10th cycle, the
normalized rate constant is ca. 0.9, indicating that the
NiCo2O4/Ti catalyst still retains 90% of its original activity.
Our NiCo2O4/Ti catalyst exhibits much improved durability
Fig.
5 (a) The accumulated volume of generated hydrogen vs.
reaction time at different reaction temperatures; (b) the logarithm of
rate constants vs. the reciprocal of reaction temperatures; catalyst
dosage = 2.4 mg.
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Table 2 Comparison of TOF and Ea of different catalysts in AB hydrolysis
TOF (mol H2 min−1
Catalysts
(mol cat.)−1
)
Ea (kJ mol−1
)
Ref.
Pt cube/CeO2/RGO
Pt NPs/CeO2/RGO
Ru@Al2O3
Ru nanoparticles
Ru/graphene
Pd-CDG
48.0
30.6
39.6
21.8
100
15.5
8.3
135
50.0
125
28.7
37.5
22.7
6.3
13.8
5.3
10.0
16.4
10.9
19.6
—
—
48
27.5
11.7
—
30
30
31
10
32
33
34
35
36
15
14
37
12
38
39
40
21
41
42
43
PdIJ0)–HAP
55
RuCu/graphene
PtPt nanocubes
Pt–Pd@PVP
Ni30Pd70/RGO
Pd@Co/graphene
Co35Pd65/C
Ag–Pd/PAN
Co/graphene
Co–P/TiO2
Electroplated Co–P
CoNi/graphene
Ni nanoparticles
Nanoporous Ni
spheres
30.59
21.8
51.7
45
—
27.5
—
32.75
48.1
22
13.49
—
Fig. 7 High-magnification SEM image of NiCo2O4/Ti after 10 cycles
(catalyst dosage = 2.4 mg).
dispersion will cause the drop of the activity of the cata-
lysts.18 Also, under the drive of high surface energy, the fine
powdery particles are very likely to aggregate during the pro-
cess of usage, isolation and desiccation, which will result in a
remarkable loss of the catalytic activity in the long-term run.
In contrast, the NiCo2O4 nanosheets in the array are
immobilized on the Ti substrate. So, they are exempt from
the activity drop caused by the migration and aggregation of
the NiCo2O4 nanosheets. Besides, the alteration of shapes
and morphology of the nanocatalysts during the catalytic pro-
cess can also influence their catalytic performance.29 To clar-
ify this, the SEM image of NiCo2O4/Ti after 10 cycles of cata-
lytic reaction is shown in Fig. 7, demonstrating that there
were no remarkable morphology changes in the NiCo2O4/Ti
catalyst. Also, the XRD pattern of NiCo2O4/Ti after 10 cycles is
similar to that of the fresh NiCo2O4/Ti. These observations
hint that our NiCo2O4/Ti catalyst is stable and reusable in the
reaction of AB hydrolysis.
27
Ni NPs@3D GFs
Co film
Co–B film
41.7
6.6
21.6
50.1
—
59
34
17.5
19
16
20
NiCo2O4
This work
The TOF values were either directly taken or calculated from the data
given in respective references.
and reusability in contrast to recently reported nanocatalysts,
such as Co–graphene (40% activity loss after 5 cycles39),
CoNi@graphene (56% activity loss after 5 cycles41), Pd–Pt@PVP
(39% activity loss after 5 cycles15), Pt/RGO (54% activity loss
after 5 cycles30), Ru/g-C3N4 (50% activity loss after 4 cycles44),
Co film (40% activity loss after 6 cycles16), and intrazeolite
Co(0) nanoclusters (31% activity loss after 5 cycles18).
For those nanocatalysts in the form of powder, the mate-
rial loss during the process of separation, drying and re-
Conclusions
In summary, the array of NiCo2O4 nanosheets with a thick-
ness of about 20 nm supported on Ti substrate (NiCo2O4/Ti)
was successfully prepared. It has been demonstrated that the
as-prepared NiCo2O4/Ti catalyst exhibits high catalytic perfor-
mance in the hydrolysis of AB for hydrogen production. The
TOF value of NiCo2O4/Ti is 50.1 mol H2 min−1 (mol cata-
lyst)−1. To the best of our knowledge, this value is higher than
those of the other noble-metal-free catalysts in the literature.
The apparent activation energy of AB hydrolysis in the pres-
ence of the NiCo2O4/Ti catalyst is as low as ca. 17.5 kJ mol−1.
More importantly, the NiCo2O4/Ti catalyst still maintains
about 90% of its original catalytic activity after 10 cycles,
exhibiting significantly improved durability and the reusabil-
ity compared with many other nanocatalysts reported in the
literature. Its high catalytic activity and low-cost, together
with its good durability and reusability enable NiCo2O4/Ti to
be a strong catalyst candidate towards the AB hydrolysis for
hydrogen generation in practical applications.
Fig. 6 Normalized rate constants and conversion at different cycles
(catalyst dosage = 2.4 mg).
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Acknowledgements
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This work was financially supported by the National Natural Sci-
ence Foundation of China (No. 51001052), the Natural Science
Foundation of Guangdong Province (No. S2013010015681),
the High-level Talent Project of the University in Guangdong
Province (184), the Characteristic and Innovative Project of
the Education Department of Guangdong Province (No.
2014KTSCX177) and the Foundation for Distinguished Young
teachers in Higher Education of Guangdong (No. Yq2013154).
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