ZIF-8 immobilized nickel nanoparticles: Highly effective catalysts for hydrogen generation from hydrolysis of ammonia borane

Article abstract of DOI:10.1039/c2cc17302f

Highly dispersed Ni nanoparticles have been successfully immobilized by the zeolitic metal-organic framework ZIF-8 via sequential deposition-reduction methods, which show high catalytic activity and long durability for hydrogen generation from hydrolysis

Full text of DOI:10.1039/c2cc17302f

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ZIF-8 immobilized nickel nanoparticles: highly effective catalysts for
hydrogen generation from hydrolysis of ammonia boranew
Pei-Zhou Li,ab Kengo Aranishiab and Qiang Xu*ab
Received 23rd November 2011, Accepted 10th February 2012
DOI: 10.1039/c2cc17302f
Highly dispersed Ni nanoparticles have been successfully immobilized
by the zeolitic metal–organic framework ZIF-8 via sequential
deposition–reduction methods, which show high catalytic activity
and long durability for hydrogen generation from hydrolysis of
aqueous ammonia borane (NH3BH3) at room temperature.
The safe and efficient storage of hydrogen is widely recognized
as one of the most challenging technologies in the transfor-
mation to a hydrogen-powered society for a long-term
solution to current energy problems.1–4 With a high hydrogen
content, ammonia borane (AB; NH3BH3) has become an
attractive candidate for chemical hydrogen storage.2–4 The
Scheme 1 Schematic illustration of preparation of Ni/ZIF-8 from
hydrogen stored in AB can be released through either
pyrolysis2 or hydrolysis.3,4 Catalytic hydrolysis can generate
(a) ZIF-8 via (b) Ni(cp)2/ZIF-8, and (c) catalytic hydrogen generation
from hydrolysis of aqueous AB over Ni/ZIF-8.
3 mol of hydrogen per mol of AB at room temperature, which
Taking into account the high chemical (highly stable in AB
presents a high hydrogen capacity of up to 9 wt% of the
aqueous solution) and thermal stability (550 1C in N2) and large
surface area, ZIF-8 (Zn(mim)2, mim = 2-methylimidazolate), a
starting materials (AB and H2O), making itself an effective
approach for hydrogen release from AB. So far many catalyst
zeolite-type MOF, becomes the most promising candidate as an
systems have been tested for hydrogen generation from the
hydrolysis of AB,3,4 among which platinum shows the highest
effective matrix to control the particle size of metal nanoparticles
activity.3b For the practical application of this system, it is still
(NPs) and improve their catalytic surface area for hydrolysis of
AB.12 Herein, we report successful preparation and excellent
essential to develop efficient and economical catalysts and
catalytic activity of highly dispersed Ni NPs immobilized by
further improve the kinetic properties via controlling the
ZIF-8, Ni/ZIF-8, which presents the first successful example of
particle size and increasing the surface area of acquired non-
noble metal catalysts, such as Ni nano-catalysts.4
MOF-supported metal catalysts for the hydrogen generation from
hydrolysis of AB.
On the other hand, due to high porosity, large surface area and
For preparing Ni/ZIF-8 (Scheme 1), a small volatile molecule
become highly promising functional hybrid materials,5–11
nickelocene (Ni(cp)2) was used as the precursor for Ni. The samples
of ZIF-8 supported Ni(cp)2 (Ni(cp)2/ZIF-8) were synthesized via
recognition and separation,7 and drug delivery and medical
two approaches: chemical vapor deposition (CVD) and chemical
chemical tunability, metal–organic frameworks (MOFs) have
especially in the application of gas sorption and storage,6 molecule
liquid deposition (CLD).13–15 In the CVD method the samples
were prepared by heating the Ni(cp)2 and ZIF-8 in separated small
glass vessels in a Schlenk tube.14 In the CLD method the samples
were prepared by mixing the diethylether solution of Ni(cp)2 and
the methanol suspension of ZIF-8 and removing the solvents via
volatilization.15 CVD-Ni/ZIF-8 (1) and CLD-Ni/ZIF-8 (2) with
different Ni loadings were obtained by reducing the above inter-
mediate samples, CVD-Ni(cp)2/ZIF-8 (3) and CLD-Ni(cp)2/ZIF-8
(4), in a H2/Ar flow (50 mol% H2, 50 mL minÀ1) at 300 1C for
3 hours.
imaging,8 as well as for heterogeneous catalysis.9 Although
MOF based catalyst preparation10 and pyrolysis11 for hydrogen
release from AB have been reported, the application of MOFs in
the catalytic hydrolysis reaction system is still a challenging task
due to the instability of most of the MOFs in aqueous solution.
a National Institute of Advanced Industrial Science and Technology
(AIST), Ikeda, Osaka, Japan. E-mail: q.xu@aist.go.jp;
Fax: +81-72-751-9629; Tel: +81-72-751-9562
b Graduate School of Engineering, Kobe University, Nada Ku,
Kobe, Hyogo, Japan
The catalytic activities on hydrolysis of aqueous AB were
tested for the prepared samples. Among the samples of 1 with
different Ni loadings (1a, 19.0 wt%; 1b, 16.7 wt%; 1c, 8.6 wt%;
determined by ICP),13 1a has the highest activity, with which the
w Electronic supplementary information (ESI) available: Experimental
procedures, N2 physisorption, PXRD, XPS, TEM, catalytic activity
and durability/stability of prepared samples. See DOI: 10.1039/
c2cc17302f
c
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Chem. Commun., 2012, 48, 3173–3175 3173

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Fig. 1 Hydrogen generation from hydrolysis of aqueous AB (2 mmol
in 2 mL water) in the presence of (a) 1a (10 mg, Ni/AB = 0.016) and
(b) 2a (10 mg, Ni/AB = 0.019) at room temperature.
reaction can be completed (H2/AB = 3.0) in 13 min (Ni/AB =
0.016) (Fig. 1a), giving a turnover frequency (TOF) value of
14.2 mol of H2Á(mol of Ni)À1ÁminÀ1. Among the samples of 2
with different Ni loadings (2a, 22.0 wt%; 2b, 14.3 wt%; 2c,
8.4 wt%),13 2a has the highest activity, with which the reaction
can be completed in 19 min (Ni/AB = 0.019) (Fig. 1b), giving a
TOF value of 8.4 mol of H2Á(mol of Ni)À1ÁminÀ1. As a control
experiment, 9.0 mg of NiCl2Á6H2O, corresponding to 2.2 mg of
Ni, was reduced using NaBH4 as reductant to produce Ni NPs
without ZIF-8 as support, with which the complete hydrogen
release from the same reaction system needed more than
50 min.13 Moreover, it is noteworthy that both the turnover
frequencies for 1a and 2a are among the highest values for the Ni
nano-catalysts ever reported,4 which indicates that the frame-
work of ZIF-8 has a high performance for effectively immobili-
zing the Ni NPs, preventing them from aggregation, and
improving the catalytic surface area of the Ni NPs.
Fig. 2 (a) N2 adsorption isotherms (77 K) of 1a and 3a. (b) Powder
XRD patterns of ZIF-8, 1a before and after catalytic AB hydrolysis,
and 3a.
that the framework of ZIF-8 effectively controlled the Ni NPs
into small sizes in both 1 and 2.
X-Ray photoelectron spectroscopy (XPS) with Ar etching
was applied to 1a and 2a. For both samples, well-defined
peaks with binding energies of 1022.7 and 1045.9 eV were
observed for the 2p3/2 and 2p1/2 levels, respectively, of the
Zn2+ ion in the ZIF-8 framework and well-defined peaks with
binding energies of 852.7 and 870.2 eV were detected for the
2p3/2 and 2p1/2 levels, respectively, of metallic Ni0 of the Ni
NPs.13,16 Even after Ar etching as long as 240 min, the Ni0
species retain their intensity, indicating the highly uniform
dispersion of Ni NPs in both 1a and 2a.
The durability/stability is very important for the practical
application of catalysts. We tested the durability of the as-prepared
1a and 2a by adding additional equivalents (2 mmol) of AB under
ambient atmosphere. There was no significant decrease in catalytic
activity even after 5 runs of hydrolysis reactions for both
catalysts.13 The high activity and long durability/stability of 1a
and 2a should be due to the fact that the highly dispersed Ni NPs
have been effectively immobilized by the frameworks of ZIF-8
(vide infra).
The morphologies of ZIF-8 immobilized Ni NPs in 1a and
2a were further characterized by transmission electron micro-
scopic (TEM) measurements. Both bright-field TEM and
high-angle annular dark-field scanning TEM (HAADF-
STEM) images show that the ZIF-8 immobilized Ni NPs were
highly dispersed and controlled in small sizes in both 1a and
2a. The mean diameters were 2.7 Æ 0.7 and 4.5 Æ 1.0 nm for 1a
and 2a, respectively (Fig. 3 and Fig. S17, ESIw).13 It should be
pointed out that a slight aggregation (Fig. 3b) was also
observed for the sample of 2a, corresponding to the observation
that the catalytic activity of 2a was lower than that of 1a. The
TEM results are in good agreement with PXRD observations,
and provide direct evidence that the highly dispersed Ni NPs
have been effectively immobilized by ZIF-8 and controlled in
small sizes, and therefore exhibit high catalytic activity and
durability for hydrolysis of aqueous AB.
N2 sorption and powder X-ray diffractions (PXRD) were
measured for the samples of 1–4. Appreciable decreases in N2
sorption as compared with pristine ZIF-8 were observed for all
the Ni-loaded samples (Fig. 2a),13 indicating that Ni NPs or
the related precursor Ni(cp)2 were dispersed to the frameworks
of ZIF-8 in all the samples. The PXRD patterns of all the
prepared samples, and also the samples of 1a and 2a after the
hydrolysis of AB exhibited that the main diffractions were well
consistent with those of pristine ZIF-8, indicating that the
framework of ZIF-8 was maintained well in the whole process
of catalyst preparation and catalytic hydrolysis of AB. Moreover,
in the PXRD patterns, no visible diffraction of Ni was detected
for 1 (Fig. 2b), while two small broad peaks around 44.31 and
51.61 due to Ni0 were detected for the samples of 2,4a,13 indicating
As suggested previously, the catalytic process takes place on
the metal surfaces where activated complex species might be
formed via interactions between the NH3BH3 molecule and the
metal particle surface,4a and therefore finding a method to
c
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