A noble-metal-free catalyst derived from Ni-Al hydrotalcite for hydrogen generation from N2H4·H2O decomposition

Article abstract of DOI:10.1002/anie.201201737

Free of nobility: A supported nickel catalyst derived from Ni-Al hydrotalcite is a promising candidate to replace noble metals for hydrogen generation from the catalytic decomposition of N₂H ₄·H₂O under ambient conditions. The catalyst produces H₂ at 30 °C in 93 % selectivity. The catalytic function is due to the cooperation of highly dispersed Ni nanoparticles and strong basic sites located nearby. Copyright

Full text of DOI:10.1002/anie.201201737

Angewandte
Chemie
DOI: 10.1002/anie.201201737
Heterogeneous Catalysis
A Noble-Metal-Free Catalyst Derived from Ni-Al Hydrotalcite for
Hydrogen Generation from N2H4·H2O Decomposition**
Lei He, Yanqiang Huang, Aiqing Wang, Xiaodong Wang, Xiaowei Chen, Juan Josꢀ Delgado, and
Tao Zhang*
Storing hydrogen safely and efficiently is one of the major
technological barriers preventing the widespread application
of hydrogen-fueled cells, such as proton exchange membrane
fuel cells (PEMFCs). Hydrous hydrazine (N2H4·H2O) is
considered as a promising liquid hydrogen storage material
owing to the high content of hydrogen (7.9%) and the
advantage of CO-free H2 produced.[1] In particular, hydrous
hydrazine offers great potential as a hydrogen storage
material for some special applications, such as unmanned
space vehicles and submarine power sources, where hydrazine
is usually used as a propellant.
employed as catalysts for this reaction. However, the nano-
particles were only active at 708C, and addition of 0.5 molLꢀ1
NaOH was necessary for the high selectivity. Moreover, the
practical application of colloidal nanoparticles will raise
significant problems, such as mass production, handling,
stability, separation, and recyclability. Therefore, from the
viewpoint of practical applications, a supported base metal
catalyst is a preferred choice owing to its low cost, good
mechanical stability, and easy separation from the reaction
medium.
Herein, using a Ni-Al hydrotalcite-like compound (Ni-Al-
HT) as the precursor, we obtained a highly dispersed nickel
catalyst that presented 100% conversion of N2H4·H2O and up
to 93% selectivity to H2 for the decomposition of N2H4·H2O
at ambient temperature. To our knowledge, this is the first
report in which supported base metal catalysts show such high
selectivity towards the formation of H2.
The decomposition of hydrazine proceeds by two typical
reaction routes:[2]
H2NNH2 ! N2ðgÞ þ 2 H2ðgÞ
ð1Þ
ð2Þ
3 H2NNH2 ! 4 NH3ðgÞ þ N2ðgÞ
It is well-known that supported noble metal catalysts,
especially iridium catalysts, are very active for the decom-
position of hydrazine. Compared with Ir, Ni is less active.[5]
Accordingly, to obtain a high activity over Ni catalysts, a very
high loading of Ni is required while maintaining a high degree
of dispersion. Hydrotalcite-like compounds have been dem-
onstrated to be excellent precursors for the preparation of
highly dispersed and high-loading metal catalysts.[6] Herein,
we synthesized binary Ni-Al-HTwith interlayer CO32ꢀ anions
by a co-precipitation method.[7] After reduction in a H2
atmosphere at 5008C, the sample was transformed into
a highly dispersed Ni/Al2O3 nanocatalyst (denoted as Ni-
Al2O3-HT). The molar ratio of Ni to Al, determined by ICP
analysis, was 3:1. The BET surface areas of the Ni-Al-HT
precursor and the derived Ni-Al2O3-HT catalyst were
248 m2 gꢀ1 and 179 m2 gꢀ1, respectively. The reduced Ni-
Al2O3-HT catalyst was then transferred into deionized
water without exposure to air and used for reaction without
further treatment. The reaction was initiated by introducing
N2H4·H2O into the reactor containing the catalyst. Gaseous
products released during the reaction were measured volu-
metrically and analyzed on-line with a gas chromatograph.[8]
The selectivity to H2 was evaluated based on Equations (1)
and (2). For comparison, a traditional supported Ni/Al2O3
catalyst was also prepared by the impregnation method with
the same amount of Ni loading (denoted as 78 wt% Ni/Al2O3-
IMP). The synthesis details can be found in the Supporting
Information.
Reaction (2) not only decreases the yield of H2 but also
complicates the separation process of products, because the
ammonia by-product would poison the Nafion membrane and
the fuel-cell catalysts. Thereby, it is of crucial importance to
develop a highly selective catalyst over which the reaction
proceeds only by pathway (1) at low temperatures. To this
end, Xu and co-workers[3] synthesized a series of nickel-
containing bimetallic nanoparticles, including Ni-Rh, Ni-Pt,
and Ni-Ir, which showed high H2 selectivity at room temper-
ature. Nevertheless, the incorporation of noble metals to
nickel greatly increased the cost of catalysts. In a subsequent
study by Xu and co-workers,[4] Ni-Fe nanoparticles were
[*] L. He, Dr. Y. Huang, Dr. A. Wang, Dr. X. Wang, Prof. T. Zhang
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences
Dalian, 116023 (China)
E-mail: taozhang@dicp.ac.cn
Homepage: http://www.taozhang.dicp.ac.cn
L. He
Graduate University of Chinese Academy of Sciences
Beijing (China)
Dr. X. Chen, Dr. J. J. Delgado
Departamento de Ciencia de los Materiales e Ingenierꢀa Metalfflrgica
y Quꢀmica Inorgµnica Facultad de Ciencias, Universidad de Cµdiz
Puerto Real, Cadiz (Spain)
[**] The authors thank Prof. Qiang Xu and Prof. Jun Li for helpful
discussions. This work was supported by the National Natural
Science Foundation of China (21076211, 21103173, 21176235).
Xiaowei Chen is grateful for the “Ramón y Cajar” program from the
Spanish Ministry of Science and Innovation.
Powder X-ray diffraction patterns (Figure 1a) illustrate
the crystalline nature of the layered double hydroxide (LDH)
structure of hydrotalcite. No other crystalline phases were
detected, thus suggesting the high purity of the precursor.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201737.
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Figure 1. a) XRD patterns of: (1) Ni-Al-HT structure; (2) Ni-Al2O3-HT
reduced at 5008C. b) HAADF image for Ni-Al2O3-HT reduced at 5008C.
c) HRTEM image (inset: SAED) for a single particle. d) Histogram of
the Ni particle size distribution.
After reduction, the major diffraction peak with a 2q value of
44.58 can be indexed to the (111) plane of fcc Ni (PDF number
01-087-0712). The average particle size of Ni is around 4 nm,
based on the Scherrer equation, indicating that the Ni-Al-HT
precursor produces a well-dispersed Ni-Al2O3-HT catalyst,
even with a very high Ni loading of around 78 wt%. Fig-
ure 1b,c show representative HAADF-STEM and HRTEM
images of the Ni-Al2O3-HT catalyst derived from hydro-
talcite. The well-dispersed Ni nanoparticles are surrounded
by amorphous alumina (Supporting Information, Figure S1),
suggesting they are strongly interacting with the alumina
support. This can be further confirmed by the results of H2-
TPR measurement (Supporting Information, Figure S2). The
reduction temperature of Ni-Al-HT is notably higher than
that of Ni/Al2O3-IMP, indicating a much stronger interaction
of the Ni component with Al2O3 in the former catalyst. The
corresponding digital diffraction pattern demonstrates the
crystalline nature of these Ni metal nanoparticles. As shown
in the size distribution histogram (Figure 1d), most of the Ni
particles are in the range of 3–9 nm, and no particles larger
than 10 nm were observed. Among them, 64% of Ni particles
are in the range of 3–5 nm. In contrast, the average size of Ni
particles of the 78 wt% Ni/Al2O3-IMP catalyst is around
38 nm (Supporting Information, Figure S3). The H2 chem-
isorption measurement also demonstrates that the Ni-Al2O3-
HT has a much larger H2 uptake than the 78 wt% Ni/Al2O3-
IMP catalyst (702 mmolgꢀ1 vs. 124 mmolgꢀ1), which is in
agreement with the conclusions from XRD and HAADF-
STEM.
Figure 2. a) n(N2+H2)/n(N2H4) versus time for Ni-Al2O3-HT and
78 wt% Ni/Al2O3-IMP at different reaction temperatures: (1) Ni-Al2O3-
HT at 508C; (2) Ni-Al2O3-HT at 308C; (3) 78 wt% Ni/Al2O3-IMP at
308C. b) n(N2+H2)/n(N2H4) versus time for Ni-Al2O3-HT at 508C: (1)
the second time; (2) the fifth time; (3) the tenth time. Inset: magnetic
separation of the catalyst after reaction.
tivity (Figure 2a); the reaction was complete within 70 min at
308C and the H2 selectivity was up to 93% (Table 1). In
contrast, the 78 wt% Ni/Al2O3-IMP catalyst exhibited a much
lower activity, with a total reaction time of 440 min. This
result is consistent with the poor dispersion of Ni. It should be
noted that the H2 selectivity was only 66%. When the reaction
temperature was raised to 508C, N2H4·H2O was converted
completely within 13 min over the Ni-Al2O3-HT catalyst and
the selectivity still maintained at 93%. Gas chromatographic
analysis reveals that the H2/N2 molar ratio was 1.95 on the Ni-
Al2O3-HT and 1.72 on the 78 wt% Ni/Al2O3-IMP, respec-
tively, with a corresponding selectivity of 93% and 67%
(Supporting Information, Figure S4). This ratio remained
constant during the whole reaction process, which is in good
agreement with H2 selectivity obtained from the volumetric
observation. On further increasing the reaction temperature
from 508C to 808C, the reaction was complete within 5 min
but the selectivity was reduced to 82% (Supporting Informa-
tion, Table S1), demonstrating that a higher temperature was
unfavorable for Reaction (1) but the reaction took place
faster than that at lower temperatures. The apparent activa-
tion energy for the reaction was about (49.3 ꢁ 3.2) kJmolꢀ1
(Supporting Information, Figure S5), which is very similar to
the previously reported values for the catalytic decomposition
The catalytic performance of the Ni-Al2O3-HT catalyst in
N2H4·H2O decomposition for H2 generation was examined
and compared with the 78 wt% Ni/Al2O3-IMP sample. The
Ni-Al2O3-HT catalyst shows much higher activity and selec-
2
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Table 1: Ni particle size after reduction, catalytic activity and selectivity, and amount of CO2 desorbed during TPD of the corresponding Ni-Al2O3-HT
and Ni/Al2O3-IMP catalysts.
Catalysts
Reaction
temperature
[8C]
Average
size of Ni
[nm]
Reaction
time[d]
[min]
Selectivity
[%]
Total amount of
CO2 desorption
[mmol(g Al2O3)ꢀ1
Strong
Weak
basic sites
basic sites
]
[mmol(gAl2O3)ꢀ1
]
[mmol(gAl2O3)ꢀ1
]
Ni-Al2O3-HT
Ni-Al2O3-HT
50
30
30
30
30
30
30
4.0
4.0
3.2
38.2
4.0
–
13
70
64
440
160
86
93
93
92
66
59
96
76
414
414
350
104
166
998
653
328
328
259
33
48
845
376
86
86
91
Ni-Al2O3-HT (2:1)[a]
78 wt% Ni/Al2O3-IMP
20 wt% Ni/Al2O3-IMP
K-Ni-Al2O3-HT[b]
71
118
153
277
K-20 wt% Ni/Al2O3-IMP[c]
–
290
[a] The molar ratio of Ni/Al was 2:1. [b] The mass percent of K for K-Ni-Al2O3-HTwas 0.23%. [c] The mass percent of K for K-20 wt% Ni/Al2O3-IMP was
2.92%. [d] The time for complete conversion of N2H4·H2O.
of gaseous N2H4.[9] The reaction rate per unit mole of active
metal over this Ni-Al2O3-HT catalyst was about 2.0 hꢀ1, which
is even comparable to the NiPt alloy nanoparticles.[3d] The
recyclability of the Ni-Al2O3-HT catalyst was also tested.
There was no significant decrease in H2 selectivity from the
second to tenth runs (Figure 2b), though the rate of reaction
decreased slightly. The slight decrease in reaction rate is due
to the formation of small amounts of ammonia rather than the
growing crystallite size. Another advantage for potential
application is that the catalyst is easily separated from the
reaction medium owing to the magnetic property of Ni
particles (inset of Figure 2b).
This is the first report that Ni catalyst can catalyze the
complete decomposition of N2H4·H2O at room temperature.
The freshly reduced Ni-Al2O3-HT catalyst exhibited desired
activity and selectivity for N2H4·H2O decomposition, which
shows the potential for it to be a candidate to replace noble
metals for this reaction. Moreover, the passivated catalyst was
totally inactive for this reaction at 308C. An IR spectrum of
adsorbed CO (Supporting Information, Figure S6) shows that
for the fresh catalyst, there is a main band appearing at
2059 cmꢀ1, which is typical for CO linear adsorption on
metallic Ni; however, there is no CO adsorption observed on
the passivated surface, which is probably due to surface
oxidation of Ni particles. This result supports the idea that the
high catalytic activity of this Ni-Al2O3-HT catalyst originates
from the metallic Ni.
range from 3 to 9 nm, while only the reaction rate increased
with the reducing of Ni particle size.
A quantitative measure of the number and strength of
basic sites was obtained by a CO2-TPD experiment (Support-
ing Information, Figure S7 and Table 1). The number of basic
sites on Ni-Al2O3-HT was clearly larger than that on the
impregnated sample. In fact, there are two kinds of basic sites,
strong and weak, and the major difference lies in the number
of strongly basic sites. By comparing 20 wt% Ni/Al2O3-IMP
and Ni-Al2O3-HT catalysts, which have quite different H2
selectivity (Supporting Information, Figure S8), we found that
the latter catalyst contained many more strong basic sites.
Therefore, it is reasonable to speculate that the strong basic
sites contribute to the increased H2 selectivity. To further
investigate the influence of the basicity of the catalyst, a small
amount of potassium was deliberately introduced to both the
Ni-Al2O3-HT and Ni/Al2O3-IMP catalysts. As expected, the
K-promoted catalyst with stronger basicity did exhibit an
increase of H2 selectivity from 93% to 96% for Ni-Al2O3-HT
and 59% to 76% for Ni/Al2O3-IMP. Noticeably, with
decreased Ni loading, there was also an increase in the
weak basicity of the sample owing to the increased content of
Al2O3, but this was not accompanied by increase in selectivity.
The decomposition reaction of N2H4·H2O consists of
ꢀ
several elementary reactions, including the cleavage of N N
ꢀ
ꢀ
and N H bonds. If the N N bond firstly breaks to produce
CNH2 radical, it is much easier to produce ammonia instead of
ꢀ
It should be noted that a much lower H2 selectivity (only
66%) was observed over the 78 wt% Ni/Al2O3-IMP catalyst.
We tentatively attribute this to Ni particle size difference
(4 nm versus 38 nm), considering that Ni catalysts are
hydrogen because further N H bond cleavage needs more
energy than the combination of CNH2 and HC to form NH3.[11]
The Ni-Al2O3-HT catalyst exhibits much higher selectivity to
H2, which is quite different from other metal catalysts (10%
for cobalt, 5% for iridium). This unique selectivity to H2 can
ꢀ
structure-sensitive for N H cleavage in NH3 decomposi-
tion.[10] To confirm this proposition, we prepared a Ni/Al2O3-
IMP catalyst with a mean particle size of 4 nm by lowering the
metal loading to 20 wt%. However, the selectivity to H2 on
the impregnated sample was still very low (around 59%),
which is even lower than the 78 wt% Ni/Al2O3-IMP catalyst,
indicating that the difference in Ni particle size may not be the
only reason for the H2 selectivity. In fact, by varying the molar
ratio of Ni/Al and reduction temperature, the particle sizes of
Ni-Al2O3-HT catalysts can be tuned (Supporting Information,
Table S2). Interestingly, there were no significant changes in
the selectivity to H2 when varying the Ni particles size in the
be explained by facilitating the dissociation of N H bond
ꢀ
over Ni-based catalyst.[5b,12] KOH has been reported to be
ꢀ
a promoter for selective cracking of the N H bond in N2H4
decomposition.[2e,4] Therefore, the stronger basicity of the Ni-
Al2O3-HT leads to a highly selective catalyst for production of
H2 from the decomposition of N2H4·H2O. On the other hand,
the selectivity of the Ni-Al2O3-HT was significantly higher
than that of the K-promoted Ni/Al2O3-IMP sample, even
though the latter catalyst contained large amounts of strong
basic sites. This may be related to the strong metal support
interaction (SMSI), as expected from the peculiar textural
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property that is typical of hydrotalcite-derived material.
Therefore, it is likely that the high H2 selectivity of Ni-
Al2O3-HT originates from the cooperative action of the small
Ni particle size and strong basic sites located nearby.
In summary, a supported Ni catalyst with a high loading
and good dispersion, prepared by using Ni-Al hydrotalcite-
like compounds as precursors, exhibits 100% conversion and
93% H2 selectivity for the decomposition of N2H4·H2O at
room temperature. This unique catalysis is due to the
cooperation of Ni nanoparticles and strong basic sites. This
Ni-Al hydrotalcite derived catalyst could be a promising
candidate to replace noble metals for the catalytic decom-
position of N2H4·H2O and for hydrogen generation under
ambient conditions.
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Received: March 3, 2012
Published online: && &&, &&&&
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Keywords: basicity · hydrogen storage ·
hydrazine decomposition · Ni-Al hydrotalcite
.
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Heterogeneous Catalysis
Free of nobility: A supported nickel
catalyst derived from Ni-Al hydrotalcite is
a promising candidate to replace noble
metals for hydrogen generation from the
catalytic decomposition of N2H4·H2O
under ambient conditions. The catalyst
produces H2 at 308C in 93% selectivity.
The catalytic function is due to the
cooperation of highly dispersed Ni nano-
particles and strong basic sites located
nearby.
L. He, Y. Huang, A. Wang, X. Wang,
X. Chen, J. J. Delgado,
T. Zhang*
&&&& — &&&&
A Noble-Metal-Free Catalyst Derived from
Ni-Al Hydrotalcite for Hydrogen
Generation from N2H4·H2O
Decomposition
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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