Bimetallic nickel-rhodium nanoparticles supported on zif-8 as highly efficient catalysts for hydrogen generation from hydrazine in alkaline solution

Article abstract of DOI:10.1002/cctc.201402353

Highly dispersed bimetallic Ni-Rh nanoparticles with an average diameter of (1.2±0.2) nm were successfully deposited on the metal-organic framework (MOF) ZIF-8 by using a simple liquid impregnation method. These catalysts were composition dependent toward the dehydrogenation of hydrazine in alkaline solution, whereas Ni₆₆Rh₃₄@ZIF-8 exhibited the highest catalytic activity among all the catalysts tested with a turnover frequency value of 140 h^-1 and 100% hydrogen selectivity at 50C. The excellent catalytic performance may be caused by the synergistic molecular-scale alloying effect of the bimetallic Ni-Rh nanoparticles on ZIF-8 and the promotion effect of ZIF-8. The development of high-performance catalysts by utilizing MOFs as a novel porous catalyst support to control the limited growth of metal nanoparticles may promote the application of hydrous hydrazine as a promising chemical hydrogen-storage material and open up new opportunities to use MOF-supported metal nanoparticles for more applications.

Full text of DOI:10.1002/cctc.201402353

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DOI: 10.1002/cctc.201402353
Bimetallic Nickel–Rhodium Nanoparticles Supported on
ZIF-8 as Highly Efficient Catalysts for Hydrogen
Generation from Hydrazine in Alkaline Solution
[
a]
[a]
[a]
[c]
[a]
[a, b]
Bingquan Xia, Nan Cao, Hongmei Dai, Jun Su, Xiaojun Wu, Wei Luo,*
and
[a]
Gongzhen Cheng
Highly dispersed bimetallic Ni–Rh nanoparticles with an aver-
age diameter of (1.2ꢀ0.2) nm were successfully deposited on
the metal–organic framework (MOF) ZIF-8 by using a simple
liquid impregnation method. These catalysts were composition
dependent toward the dehydrogenation of hydrazine in alka-
line solution, whereas Ni Rh @ZIF-8 exhibited the highest cat-
zine monohydrate more competitive than solid chemical hy-
drogen-storage materials such as sodium borohydride (NaBH4)
and ammonia borane (NH BH ) derivatives. However, from the
3
3
perspective of hydrogen-storage applications, the incomplete
decomposition to ammonia, which is toxic to fuel-cell catalysts,
by another pathway [Eq. (2)] should be avoided. Recently,
a number of noble and non-noble metal-containing mono-
66
34
alytic activity among all the catalysts tested with a turnover
ꢁ
1
[5]
frequency value of 140 h and 100% hydrogen selectivity at
08C. The excellent catalytic performance may be caused by
and bimetallic nanocatalysts have been developed. How to
5
strike a balance between cost, selectivity, efficiency, and recy-
clability, however, still remains a considerable challenge.
the synergistic molecular-scale alloying effect of the bimetallic
Ni–Rh nanoparticles on ZIF-8 and the promotion effect of ZIF-
H NNH
! N2 ðgÞ þ 2 H2 ðgÞ
2 ðlÞ
ð1Þ
ð2Þ
8
. The development of high-performance catalysts by utilizing
2
MOFs as a novel porous catalyst support to control the limited
growth of metal nanoparticles may promote the application of
hydrous hydrazine as a promising chemical hydrogen-storage
material and open up new opportunities to use MOF-support-
ed metal nanoparticles for more applications.
3
H NNH ! N2 ðgÞ þ 4 NH3 ðgÞ
2
2 ðlÞ
On the other hand, as a new class of hybrid functional mate-
rials and owing to their high porosity, large surface area, and
chemical tenability, metal–organic frameworks (MOFs) have at-
tracted growing attention in a variety of applications, such as
[6]
[7]
[8]
The search for safe and effective hydrogen-storage materials is
still one of the most challenging obstacles for the develop-
ment of hydrogen fuel-cell technology. Over the past decades,
numerous hydrogen-storage approaches have been explored,
gas storage, gas separation, heterogeneous catalysis, sens-
[9]
[10]
ing, and drug delivery.
Given their similarity to zeolites,
loading metal nanoparticles (NPs) into the pores of MOFs is ex-
pected to control the limited growth of the metal NPs in the
confined cavities and limit the migration and aggregation of
the metal NPs, and thus further increase their catalytic activity
and stability. Loading metal NPs inside the pores of MOFs is of
current interest. Until now, a number of monometallic Pd, Pt,
Ir, Au, and Ru NPs and their bimetallic alloy NPs with non-
noble metals have been successively immobilized into the
[
1]
[2]
including metal hydrides, sorbent materials, and chemical
[
3]
hydride systems. Among them, hydrous hydrazine (N H ·H O)
2
4
2
is considered as a promising chemical hydrogen material be-
cause of its high hydrogen content (8.0 wt%) and safe han-
dling, the ease with which it can be recharged, and the fact
that only nitrogen in addition to hydrogen are produced
through a complete decomposition route, as shown in Equa-
[11]
pores of MOFs. As far as we know, however, there is no
report on MOF-supported Rh-based NPs with full characteriza-
[
4]
tion (1). Moreover, hydrazine monohydrate is a liquid-phase
material that has the potential to take advantage of the exist-
ing liquid-based distribution infrastructure, which makes hydra-
[12]
tion. Herein, we report the first MOF-supported Ni–Rh bimet-
allic NPs. We study the synergistic effect of metal composition
in the MOFs for the catalytic dehydrogenation of hydrazine in
[
a] B. Xia, N. Cao, H. Dai, Dr. X. Wu, Prof. W. Luo, Prof. G. Cheng
College of Chemistry and Molecular Sciences
Wuhan University
Wuhan, Hubei 430072 (P.R. China)
E-mail: wluo@whu.edu.cn
alkaline solution. The ZIF-8 framework [Zn(MeIM) , MeIM=2-
2
methylimidazole], one of the preventative MOFs, was used as
a support because of its intersecting 3D structure, high ther-
[13]
mal and chemical stability, and large surface area. Relative to
the activity of other reported catalysts for the dehydrogena-
tion of hydrazine, the Ni Rh @ZIF-8 catalyst exhibits the high-
[b] Prof. W. Luo
Suzhou Institute of Wuhan University
Suzhou, Jiangsu, 215123 (P.R. China)
66
34
est catalytic activity with a turnover frequency (TOF) value of
ꢁ
1
[
c] Dr. J. Su
1
40 h and 100% hydrogen selectivity at 508C.
Wuhan National Laboratory for Optoelectronics
Huazhong University of Science and Technology
Wuhan, Hubei, 430074 (P.R. China)
The bimetallic Ni–Rh NPs with different Ni–Rh compositions
were successfully immobilized by ZIF-8 through a simple liquid
impregnation method. Activated ZIF-8 (100 mg) was impreg-
nated with deionized water (4 mL) containing a total of
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402353.
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0
.2 mmol of nickel chloride and rhodium chloride in different
{111} lattice spacing of face-centered cubic (fcc) Ni (0.203 nm)
and fcc Rh (0.219 nm); this suggests that Ni–Rh is formed as
an alloy structure. The EDX spectra (Figure 1d) further con-
firmed the presence of Ni–Rh. In addition, X-ray photoelectron
spectroscopy (XPS) measurements were performed to under-
stand the states of Rh and Ni that coexist in the catalyst, for
which characteristic signals of both metals were detected (Fig-
molar ratios (1:9, 2:8, 4:6, 7:3, 9:1) at 298 K for 12 h. To obtain
the NiRh@ZIF-8 catalysts, the as-synthesized samples were sep-
arated by centrifugation and dried, which was followed by re-
duction by using sodium borohydride at 273 K. The final ratio
of Ni and Rh in the catalysts was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES),
which was close to the initial loading (Table S1, Supporting In-
formation). The powder X-ray diffraction (PXRD) patterns of as-
synthesized ZIF-8, Ni@ZIF-8, Rh@ZIF-8, and Ni Rh @ZIF-8 re-
0
0
ure S3), and this demonstrated the composition of Ni and Rh
in the NiRh@ZIF-8 catalyst. The observed Ni2p3/2 and Ni2p1/2
binding energies at 856.5 and 874.6 eV, respectively, corre-
66
34
0
vealed no loss of crystallinity, as shown in Figure S1, and this
indicates that the integrity of the ZIF-8 framework was main-
tained during catalyst preparation. Furthermore, no clear dif-
fraction peaks of Ni or Rh were detected, probably as a result
of the low concentrations and small sizes of the NPs immobi-
spond to Ni , and the Rh3d5 and Rh3d3/2 binding energies at
/2
0
[14]
310.3 and 314.8 eV, respectively, correspond to Rh .
The catalytic dehydrogenation of hydrazine was performed
over all the samples at 508C in the presence of NaOH (0.5m),
as shown in Figure 2. The activities of the catalysts were
lized by ZIF-8 (see below). The N adsorption–desorption iso-
2
therms of ZIF-8 and Ni Rh @ZIF-8 are shown in Figure S2a,
66
34
and they further confirm the high stability of the porous struc-
2
ꢁ1
ture of ZIF-8. The specific areas were 1644 and 786 m g for
ZIF-8 and Ni Rh @ZIF-8, respectively. A significant decrease in
66
34
the amount of N2 adsorption and a decrease in the pore
volume (Figure S2b) of Ni Rh @ZIF-8 indicate that the cavities
66
34
of ZIF-8 were either occupied by the well-dispersed Ni–Rh NPs
or blocked by the Ni–Rh NPs. The morphology of Ni Rh @ZIF-
66
34
8
was further characterized by transmission electron microsco-
py (TEM) and energy-dispersive X-ray spectroscopy (EDX)
measurements (Figure 1). Well-dispersed Ni–Rh NPs, with an
average diameter of (1.2ꢀ0.2) nm, were encapsulated in the
cages of ZIF-8, which may explain the high catalytic activity
and durability for the dehydrogenation of hydrazine (see
below). A representative high-resolution TEM image in Fig-
ure 1c shows the d spacing of 0.215 nm, which is between the
Figure 2. Time course plots for the decomposition of hydrazine in aqueous
NaOH solution (0.5m) catalyzed by a) Ni91Rh
*), c) Ni66Rh34@ZIF-8 (~), d) Ni35Rh65@ZIF-8 (!), e) Ni14Rh86@ZIF-8 (3),
f) Rh@ZIF-8 ("), and g) Ni@ZIF-8 (^). Catalyst=0.100 g, N
·H O=0.1 mL.
9
@ZIF-8 (
&
), b) Ni82Rh18@ZIF-8
(
2
H
4
2
strongly dependent on the Ni–Rh compositions; whereas
Ni@ZIF-8 and Rh@ZIF-8 were catalytically inactive,
Ni Rh @ZIF-8 presented poor catalytic activity (only about
35
65
2
equiv. gas was released over 1 h) and low hydrogen selectivi-
ty (70%). Among all the bimetallic NiRh@ZIF-8 samples investi-
gated, Ni Rh @ZIF-8 exhibited the highest catalytic activity,
66
34
ꢁ
1
with a TOF value of 140 h at 508C, which is the highest value
among all the reported catalysts (Table 1); this highlights the
synergistic effect of molecular-scale Ni–Rh alloying composi-
tions in ZIF-8 for their catalytic activity. The H selectivity and
2
completeness of hydrazine decomposition over Ni Rh @ZIF-8
66
34
were further confirmed by mass spectrometry (Figure S4),
which indicated 100% H selectivity. Furthermore, as a control
2
experiment, the same amount of Ni Rh NPs and ZIF-8 were
66
34
synthesized and applied to the dehydrogenation of an alkaline
solution of hydrazine. As shown in Figure S5, only 2.36 equiva-
lents of gas with 76% H selectivity was released over 60 min
2
Figure 1. a–c) TEM images of Ni66Rh34@ ZIF-8 at different magnifications. The
inset in a) shows the particle-size distribution of the Ni66Rh34 NPs. d) EDX of
Ni66Rh34@ ZIF-8.
for the Ni–Rh NPs, and almost no reactivity for ZIF-8 was ob-
served in the dehydrogenation of an alkaline solution of hydra-
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pipet, and complete dissolution was achieved by sonication. The
vial was capped and placed in an oven at 1208C for 24 h. ZIF-8
crystals were collected and washed with DMF. The crystals were
stored in DMF until needed for experiments.
Table 1. Comparison of the activities of the different catalysts in the gen-
eration of hydrogen through the decomposition of H
ous solution.
2
NNH
2
·H
2
O in aque-
Catalyst
T
TOF
E
a
Ref.
ꢁ
1
ꢁ1
[K]
[h
]
[kJmol ]
Preparation of the catalyst
NiRh
Ni90Rh10
NiRh
4
298
323
298
323
323
323
303
323
298
298
303
9.6
4.5
20
140
6
6
16.5
90
28.1
4.5
2.2
–
–
–
[15]
[5f]
[5g]
this work
[4f]
[16]
[4b]
[11g]
[4a]
[5b]
Prior to metal loading, as-prepared ZIF-8 was pretreated as follows:
ZIF-8 was immersed in methanol under ambient conditions for
4
/graphene
Ni66Rh34@ZIF-8
Ni0.6Pd0.4
58.1
–
49.95
49.3
–
42.3
–
34
4
5
8
8 h and then evacuated at room temperature for over 10 h and at
73 K for 2 h to obtain optimally evacuated sample. The NiRh@ZIF-
catalysts were synthesized by a simple liquid impregnation
Ni0.99Pt0.01
NiPt0.057/Al
2
O
3
method. A typical synthesis procedure for the catalyst is described:
Activated ZIF-8 (100 mg) was mixed with RhCl ·3H O (0.02 mmol)
and NiCl ·6H O (0.18 mmol) in deionized water (10 mL). The solu-
2 2
tion was continuously stirred for 12 h at 258C to impregnate the
metal salts. After impregnation, the suspension was concentrated
under vacuum. The resulting mixture was then reduced by using
Ni80Pt20@ZIF-8
Ni0.9Pt0.1/Ce
Ni0.95Ir0.05
Ni/Al
2
O
3
3
2
2
O
3
[4b]
sodium borohydride (NaBH , 78.6 mg) with vigorous stirring at
4
zine at 508C; this is indicative of the synergetic effect of the
2
73 K to yield Ni Rh @ZIF-8. The preparations of Ni Rh @ZIF-8,
14 86 35 65
Ni–Rh NPs and the framework of ZIF-8.
Ni Rh @ZIF-8, Ni Rh @ZIF-8, Ni Rh @ZIF-8, Rh@ZIF-8, and
66 34 82 18 91 9
To obtain the activation energy (E ) of the dehydrogenation
Ni@ZIF-8 followed an analogous process.
a
of an alkaline solution of hydrazine catalyzed by Ni Rh @ZIF-
66
34
8
, reactions were performed at temperatures ranging from 50
Hydrous hydrazine decomposition test
to 808C (Figure S6), and the value of E was determined to be
5
a
ꢁ1
8.01 kJmol , which is close to the reported value (Table 1).
In a typical experiment, Ni66Rh34@ZIF-8 (100 mg) and NaOH (80 mg)
were dissolved in water (4 mL) in a two-necked round-bottomed
flask with vigorous stirring. One neck of the flask was connected to
a gas burette to monitor the volume of gas evolution, and the
other neck was used to introduce hydrazine monohydrate (0.1 mL,
Furthermore, we tested the stability of the Ni Rh @ZIF-8
66
34
catalyst in the dehydrogenation of an alkaline solution of hy-
drazine at 508C (Figure S7). The results showed that there was
no significant decrease in catalytic activity and no change in
the hydrogen selectivity even after the fifth run.
1
.96 mmol). A water bath was used to control the temperature of
the solution at 323 K. The gas released during the reaction was
passed through 1.0m HCl before it was measured volumetrically.
The selectivity towards H2 generation (X) can be calculated by
using Equation (3):
In summary, well-dispersed Ni–Rh NPs have been immobi-
lized on the framework of ZIF-8 and exhibit high catalytic ac-
tivity and durability toward hydrogen generation from aque-
ous hydrazine solution. Furthermore, it is expected that this
simple liquid impregnation synthetic method could be extend-
ed to other MOF-supported bimetallic or polymetallic metal
NPs for further application.
ð3lꢁ1Þ
X ¼
8
ð3Þ
nðH þ N Þ
2
2
l ¼
nðH NNH Þ
2
2
Experimental Section
To obtain the activation energy (E
ture was varied from 323 to 353 K (catalyst=100 mg; N H ·H O=
), the catalytic reaction tempera-
a
2
4
2
Chemicals and materials
0
.1 mL).
All chemicals were commercial and were used without further pu-
rification. Zinc nitratehexahydrate [Zn(NO ) ·6H O, Sinopharm
3
2
2
Recycle stability tests
Chemical Reagent Co., Ltd., 99%], nickel chloride hexahydrate
NiCl ·6H O, Sinopharm Chemical Reagent Co., Ltd., ꢂ99%), rhodi-
(
For the recycle stability test, the catalytic reactions were repeated
five times by adding another equivalent of hydrous hydrazine
(2 mmol) to the mixture after the previous cycle.
2
2
um chloride trihydrate (RhCl ·3H O, Adamas Reagent Co., Ltd.,
3
2
9
9%), hydrazine monohydrate (H N ·H O, TCI Shanghai Co., Ltd.,
4 2 2
>
98%), aqueous hydrofluoric acid (HF, Sinopharm Chemical Re-
agent Co., Ltd., 40%), sodium borohydride (NaBH , Sinopharm
4
Chemical Reagent Co., Ltd., 96%), and ethanol (C H OH, Sino-
Characterization
2
5
pharm Chemical Reagent Co., Ltd., >99.8%) were used as re-
ceived. We used ordinary distilled water as the reaction solvent.
The morphologies and sizes of the samples were observed by
using a Tecnai G20 U-Twin transmission electron microscope
equipped with an energy-dispersive X-ray detector at an accelera-
tion voltage of 200 kV. Powder X-ray diffraction patterns were mea-
sured by using a Bruker D8-Advance X-ray diffractometer by using
a CuKa radiation source (l=0.154178 nm) with a velocity of
Synthesis of ZIF-8
[13b]
ZIF-8 was synthesized by using a reported procedure.
Zn-
ꢁ
1
(
NO ) ·6H O (350 mg) and 2-methylimidazole (200 mg) were placed
18min . X-ray photoelectron spectroscopy measurements were
3
2
2
in a 20 mL screw-top vial and dissolved in DMF (15 mL). Three
drops of HNO3 were added to the mixture by using a Pasteur
performed with a Kratos XSAM 800 spectrophotometer. The sur-
face area measurements were performed with N adsorption–de-
2
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sorption isotherm at liquid-nitrogen temperature (77 K) after dehy-
dration under vacuum at 1208C for 12 h by using a Quantachrome
NOVA 4200e. Inductively coupled plasma atomic emission spec-
troscopy was performed with an IRIS Intrepid II XSP (Thermo Fisher
Scientific, USA). Mass spectra of the generated gases were collect-
ed by using a Ametek Dycor mass spectrometer under an argon
atmosphere.
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Published online on August 19, 2014
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