Fabrication and characterization of a novel nanoporous Co-Ni-W-B catalyst for rapid hydrogen generation

Article abstract of DOI:10.1039/c4ra12230e

A highly active nanoporous Co-Ni-W-B alloy has been prepared using chemical reduction in an ethanol solution and tested as a novel catalyst for hydrolysis of ammonia borane. Compared with the alloy prepared in an aqueous solution, the as-prepared alloy shows a much higher surface area and hydrogen generation rate. This journal is

Full text of DOI:10.1039/c4ra12230e

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Fabrication and characterization of a novel
nanoporous Co–Ni–W–B catalyst for rapid
hydrogen generation†
Cite this: RSC Adv., 2015, 5, 163
Received 12th October 2014
Accepted 24th November 2014
a
a
b
a
a
a
Cuili Xiang, Jun Cheng, Zhe She, Yongjin Zou,* Hailiang Chu, Shujun Qiu,
a
a
a
Huanzhi Zhang, Lixian Sun* and Fen Xu
DOI: 10.1039/c4ra12230e
www.rsc.org/advances
A highly active nanoporous Co–Ni–W–B alloy has been prepared these catalysts show comparable catalytic activities to the noble
11,12
using chemical reduction in an ethanol solution and tested as a novel metals.
However, the stability, surface oxidation and
catalyst for hydrolysis of ammonia borane. Compared with the alloy agglomeration of catalyst particles restrict their applications in
prepared in an aqueous solution, the as-prepared alloy shows a much the hydrogen generation reactors. Supporting the Co or Ni
higher surface area and hydrogen generation rate.
based catalyst on other substrates or introducing atomic barrier
using transition metals, such as W, Cr, Cu, Mo, have been
regarded as effective ways to improve the stabilities and
Hydrogen has been considered as a promising energy carrier
due to its high efficiency and near zero emissions. However,
developing a safe as well as an efficient method to store and
produce hydrogen is still a challenge. With a high hydrogen
capacity of 19.6 wt%, good stability and being non-toxic,
13–16
dispersions of the active phases.
This is important, as the
catalytic characteristics of these catalysts are strongly depend-
13
ing on their sizes, shapes, porosity, and preparation methods.
Nanoporous catalysts are currently under intensive inves-
tigations in the catalysis community due to their exceptional
catalytic activities and selective natures when compared to the
3 3
ammonia borane (AB, NH BH ) is one of leading candidates
1
for a hydrogen storage medium. Typically, hydrogen stored in
AB can be released by thermal dehydrogenation and hydrolysis.
Comparing to the thermal dehydrogenation, the hydrolysis is
more attractive because it can be carried out at room tempera-
8,12
corresponding bulk counterpart. The nanoporous catalysts
have advantage of large surface-to-volume atomic ratios, sizes
and shape dependent properties, and high concentration of
low coordinated active surface sites, which increase the
ture and produce H
2
without generating other unwanted
10
contact area with the reactants efficiently. In the present
2,3
byproducts. Since AB has good stability in aqueous solution
under room temperature, catalysts are oen required to accel-
erate the hydrolysis reaction by lowering the kinetic barrier in a
study, a novel method of using corresponding metal salts
suspended in ethanol solution is proposed to prepare a
nanoporous Co–Ni–W–B catalyst. The Co–Ni–W–B catalyst
formed exhibit superior catalytic activity when compared to
the ones prepared from aqueous solution, due to its high
surface area. The synthesized Co–Ni–W–B catalysts have been
tested for understanding of their hydrogen generation abilities
in AB solutions.
4
tunable manner.
5
–7
Various catalysts have been tested for AB hydrolysis,
among which noble metal such as Pt-based alloys exhibit the
high catalytic activities.
8–10
However, the high cost and low
abundance of noble metal limit their real-to-life practical
applications. Therefore, it is of great interest to develop high
performance and low cost catalysts. The catalysts based on
transition metals, such as Co or Ni, are generally used to
accelerate the hydrolysis reaction of AB due to their exceptional
The preparation of Co–Ni–W–B alloy has been achieved
using chemical reduction in ethanol solution. The metal salts
such as CoSO
limited solubility in ethanol solution and form suspension as
shown in Fig. 1a and b. Aer adding NaBH solution into the
4 2 4 2 2 4 2
$7H O, NiSO $6H O and Na WO $2H O have
4
catalytic activity and low cost. It has been demonstrated that
4
suspension, the reduction occurs immediately. The interesting
observation as shown in Fig. 1c and d is that reduced black
precipitates oat on top in the ethanol solution, while precipi-
tates from same reaction move down to the bottom when
a
Guangxi Key Laboratory of Information Materials, Guilin University of Electronic
Technology, 1# Jinji Road, Guilin, 541004 China. E-mail: zouyongjing518@126.com;
sunlx@guet.edu.cn
b
Department of Physical and Environmental Sciences, University of Toronto aqueous solution used as the reaction medium. This indicates
Scarborough, Toronto M1C 1A4, Canada
that the two precipitates have different density therefore
structures.
†
Electronic supplementary information (ESI) available: Experimental procedures
and characterization/catalytic test results. See DOI: 10.1039/c4ra12230e
This journal is © The Royal Society of Chemistry 2015
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Nitrogen adsorption–desorption isotherms of the Co–Ni–W–
B catalysts prepared in ethanol and aqueous are presented in
Fig. 2. The inset of the gure shows the pore size distributions
calculated using Barrett–Joyner–Halenda (BJH) method. The
isotherm shows a typical IUPAC type pattern with inection of
N
2
adsorbed at P/P
0
about 0.40, indicating the presence of
18
mesopores. The nanoporous Co–Ni–W–B has a large Bru-
nauer–Emmett–Teller (BET) surface area of 285.2 m g and
the average pore diameter is 2.5 nm.
2
ꢀ1
In comparison, Co–Ni–W–B alloy prepared in aqueous
2
ꢀ1
displays a BET surface area of 69.3 m g . The specic surface
areas (SSA) of Co–Ni–W–B prepared in ethanol is about 4 times
higher than that of Co–Ni–W–B prepared in aqueous, which is
due to the formation of the mesoporous structure in the Co–Ni–
W–B owers. Thus, it is expected that the porous Co–Ni–W–B
10
2
provides a higher rate of H generation in AB solution.
The X-ray diffraction (XRD) patterns of the Co–Ni–W–B
prepared by using chemical reduction method display broad
ꢁ
peak around 30–50 for the Co–Ni–W–B sample prepared in
Fig. 1 Photos (a) metal salts in ethanol; (b) metal salts in aqueous; (c)
Co–Ni–W–B prepared in ethanol; (d) Co–Ni–W–B prepared in
aqueous solution, indicating their amorphous structures
aqueous; SEM images of (e) Co–Ni–W–B prepared in aqueous, (f and (Fig. S3, ESI†). In contrast, sharper diffraction peaks are
g) Co–Ni–W–B prepared in ethanol; (h) TEM image of Co–Ni–W–B
prepared in ethanol solution.
observed for the Co–Ni–W–B prepared in ethanol, which indi-
cate that this type of Co–Ni–W–B prefers to crystallize. It is
probably because that using ethanol as the solvent enables the
transformation of Co–Ni–W–B from amorphous phase to its
The SEM image in Fig. 1e shows that the Co–Ni–W–B powder
prepared in aqueous is mainly composed of irregular aggre-
gated particles with sizes in the range from 50 to 100 nm. Due to
the exothermic nature of the reduction reaction and high
surface energy involved, these particles are mostly formed in
agglomerated state with low specic surface area. In compar-
ison, the Co–Ni–W–B prepared in ethanol presents nanoporous
crystalline phase, but the mechanism is currently unclear.
However, it has been considered that the amorphous structure
may enhance the catalytic activity due to their unique short-
range ordering but long-range disordering structure, surface
12
18
coordinating unsaturated sites and the lack of crystal defects.
The catalytic performance of the as-prepared catalyst was
determined by measuring the HG rate of AB hydrolysis (1 wt%

ower-like morphology composed of interconnected nano-
sheets. The TEM images also conrmed that the formation of
nanosheet structures in the nanoporous structure as shown in
Fig. 1h. Presumably, due to the low solubility of the metal salts
in ethanol solution, the reaction rate is limited during the
ꢁ
solution, 10 mL) at 30 C. The amount of catalysts (0.01 g)
loading was kept same for all the catalyst powders. Fig. 3 shows
the volumes of hydrogen generated by the Co–Ni–W–B catalysts
prepared from using different solutions. Hydrogen is instanta-
neously produced as soon as the AB soon come in contact with
catalysts without any induction time and reach the 100% of the
theoretical hydrogen yield for both catalysts. The required
initial stage. Aer the dropwise addition of 20 mL of NaBH4
solution, the mixture reacts vigorously and release large amount
of hydrogen gas. The hydrogen gas pushes the particle up. In
this process, the nanoporous structure is formed and hydrogen
gas is conned in the pore, which enable the precipitation

oating on the surface of the solution. However, due to the good
solubility of metal salts in the aqueous solution, the reduced
particles aggregate and sink in the water. The speculation is that
the use of ethanol as the solvent leads to the formation of
porous morphology, which could be due to the change in the
nucleation and growth of the catalyst species during the
synthesis. More investigations are required in the future to
interpret the morphological formation mechanism.
Energy dispersive X-ray (EDX) analysis indicates that the
nanoparticles prepared from ethanol solution consist elements
of Co, Ni, W and O (Fig. S2, ESI†). The observation of O element
is possibly originated from sample preparation and washing in
17
air. The composition of the nanoporous catalyst has deter-
mined by using ICP-AES measurement to be 63.52 wt% Co,
Fig. 2
inset) of Co–Ni–W–B prepared in aqueous and Co–Ni–W–B
prepared in ethanol.
2
N adsorption and desorption isotherm and pore distribution
19.94 wt% Ni, 9.36 wt% W, 7.44 wt% B, corresponding to a
(
formula of Co21Ni
7
1 13
W B .
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hydrolysis is a zero order reaction with respect to the concen-
tration of AB. The observation is consistent with precious
22
report. In order to identify the reaction order with respect to
amount of nanoporous Co–Ni–W–B catalyst, the HG rate was
measured by using different amounts of nanoporous Co–Ni–W–
B catalyst: 0.05, 0.10, 0.15, and 0.20 g. The reaction time
required to complete the reaction decreases rapidly with
increasing catalyst amount. The HG rate versus amount of
catalyst is plotted, yielding a tted straight line (Fig. S6, ESI†).
This result clearly indicates that the catalytic hydrolysis reaction
of AB is quasi rst-order with respect to the amount of catalyst.
The generation of hydrogen with the as-prepared Co–Ni–W–
B catalyst in 1 wt% AB solution at different solution tempera-
ꢁ
tures ranging from 30 to 60 C was monitored (Fig. S7, ESI†). As
Fig.
3 Hydrogen generation of AB using Co–Ni–W–B catalysts
the temperature increases, the H
W–B catalyst dramatically increases. The initial stage of H
2
generation rate of the Co–Ni–
prepared in aqueous solution and in ethanol, respectively (batch
system, 30 C, 1.0 wt% AB, 0.01 g catalyst).
ꢁ
2
generation reaction by the hydrolysis of AB was found to obey
zero-order rate law. The reaction rate can be expressed as
reaction time to complete the reaction was approximately 2.5 follows:
min in the presence of nanoporous Co–Ni–W–B prepared in
ethanol, which was much shorter than the 13 min from using
Co–Ni–W–B prepared in aqueous. The HG rates (r) were calcu-
ꢀ
ꢁ
ꢀ
E
a
r ¼ k
0
exp
RT
ꢀ
1
ꢀ1
where k
0
is the rate constant (mL min
g
), E
a
is the activation
lated by using equation: r ¼ (V/t)/m, where V is the generated
ꢀ
1
ꢀ1
13
energy (kJ mol ), R is the gas constant (8.3143 kJ (mol K) ),
and T is the reaction temperature (K). Using the zero-order rate
constant (k) at varied temperatures, an Arrhenius plot of ln r
the initial HG rate) versus the reciprocal of absolute tempera-
ture (1/T) is made. From the slope of the straight line, the
activation energy of hydrolysis reaction using the nanoporous
Co–Ni–W–B catalyst is calculated to be 21.36 kJ mol . This
value of activation energy is lower than the results reported for
the hydrolysis of AB catalyzed by the Ru/C (76.0 kJ mol ),
hydrogen volume (mL), m is the amount of catalyst (g). From
the slopes of the tting lines that were obtained from the V–t
curves, the hydrogen generation (HG) rates of Co–Ni–W–B
prepared in respective aqueous and ethanol solutions were 1430
(
ꢀ
1
ꢀ1
and 6360 mL min
g
catalyst. The nanoporous Co–Ni–W–B
catalyst from preparation in ethanol shows about 4.5 times
higher HG rate than that of the ones prepared in aqueous. The
obtained HG rate value is signicantly higher than other known
catalysts, such as Pd nanoparticles supported on MIL-101 (1008
ꢀ
1
ꢀ1
23
ꢀ1
24
ꢀ1 12
ꢀ
1
ꢀ1
8
Ni0.99Pt0.01 (49.27 kJ mol ), Co–B/SBA-15 (43 ꢂ 2 kJ mol ),
mL min
g
), in situ synthesized Fe nanoparticles (1519 mL
ꢀ1
15
ꢀ
1
ꢀ1 19
ꢀ1 ꢀ1 12
Co–Mo–B/Ni foam (44.3 kJ mol ), Cu/RGO (38.2 ꢂ 1.5 kJ
ꢀ1 27
min
g
), Co–B supported on SBA-15 (1900 mL min
g ),
), Co–W–B–P
and in situ
). These results
ꢀ
1
25
ꢀ1 26
ꢀ
1
ꢀ1 20
mol ), Ru@SiO
2
(38.2 kJ mol ), Cu–Ni (38.2 kJ mol ),
Co–P deposited on TiO2 (2002 mL min
supported on Ni foam (4000 mL min
synthesized Fe0.5Ni0.5 (5372 mL min
g
g
ꢀ
1
13
ꢀ
1
ꢀ1 13
Co–W–B/Ni foam (29 kJ mol ), nanoporous nickel spheres (27
)
ꢀ
1
28
ꢀ1 29
ꢀ
1
ꢀ1 21
ꢂ 1 kJ mol ), and comparable to Co–P (22 kJ mol ), Pt/g-
g
ꢀ
1
30
Al O , Rh/g-Al O , Ru/g-Al O (21–23 kJ mol ). As the reac-
demonstrate that nanoporous Co–Ni–W–B is a highly active
catalyst for AB hydrolysis.
2
3
2
3
2 3
tion rate is inuenced by the activation energy, nanoporous Co–
Ni–W–B catalyst fabricated in this study is shown to be one of
the best for the hydrolysis reaction.
For comparison, the ternary Co–Ni–B, Co–W–B, Ni–W–B and
binary Co–B, Ni–B were also prepared in ethanol solution with
same chemical reduction method. It can be seen that the
nanoporous Co–Ni–W–B shows highest catalytic activity when
compared to the same amount of Co–B, Ni–B, Co–Ni–B, Co–W–
B, Ni–W–B catalyst powders (0.01 g) (Fig. S4, ESI†). The HG rates
acquired with using these catalysts decrease in following order:
Co–Ni–W–B > Co–W–B > Co–Ni–B > Ni–W–B > Co–B > Ni–B. This
could be attributed to the introduction of transition metals,
which are known to effectively improve the catalytic activity for
To investigate the stability of the nanoporous Co–Ni–W–B
catalyst, the catalyst has been tested by the cyclic hydrolysis
experiments. The catalyst has been recycled aer the comple-
tion of the hydrolysis reaction, washed thoroughly with deion-
ized water and loaded into another AB solution. The
nanoporous Co–Ni–W–B catalyst shows satisfactory durability
in the cyclic usages (Fig. S8, ESI†). The catalyst preserved 87% of
its original HG rate at its tenth application, which has better
stability than the Co–W–B–P/Ni foam (68% of its original HG
3
hydrolysis of borohydride.
13
rate aer tenth usage). It has been shown that the deteriora-
tion in catalytic activity of the catalyst used for the hydrolysis of
borohydride is attributed to the enriched byproducts on the
surface of the catalyst, which decrease the accessibility of active
sites. A fast HG rate can help reducing the adsorption of the
byproducts on the surface of the catalyst because generated gas
The effect of AB concentration on the hydrogen generation
rate in the presence of nanoporous Co–Ni–W–B catalyst at 30 C
is also examined. There is no obvious change in HG with
increasing in concentration of AB. This indicates that the
hydrolysis of AB by the Co–Ni–W–B catalyst is not dependent on
the concentration of AB (Fig. S5, ESI†). It suggests that
ꢁ
15
blows away the byproducts.
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92, 68.
ꢀ
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ꢀ1
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g
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