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Thus an apparent first-order dependence on catalyst concen-
tration was observed. This observation indicates that the hy-
drogen generation rate is controlled by the surface reaction.
Thus the increased active sites on small NPs are expected to
exhibit high catalytic activity in the dehydrogenation of AB. It
has been reported that a possible mechanism for catalytic de-
hydrogenation of AB involves the following three steps: i) for-
mation of an activated complex species between AB and the
metal particle surface, ii) concerted dissociation of the BÀN
bond upon attack by a H2O molecule, and iii) hydrolysis of the
resulting BH3 intermediate to produce H2 along with the for-
showed partially segregated Fe and Ni atoms on its surface. An
investigation using a series of FeNi/CeO2 combinations with
different Fe/Ni ratios resulted in a volcano-shaped relationship.
The catalytic system described here is a powerful candidate for
a H2 generation protocol, thanks to the following advantages:
(i) no requirement of noble metals, (ii) simple work-up proce-
dures by application of an external magnet and the durability
of catalyst lifetime, (iii) superior catalytic activity to colloidal
FeNi or FeNi/SBA-15, and iv) applicability to dehydrogenation
from dimethylamine–borane ((CH3)2NHBH3).
À
mation of a BO2 ion.[18] The superior catalytic activity of FeNi/
Experimental Section
CeO2 may be attributed to the formation of highly dispersed
FeNi NPs as well as structurally distorted NiÀOÀCe and
FeÀOÀCe bonding. Such amorphous FeNi catalysts provide
a much higher concentration of surface active sites to promote
the formation of an activated complex, which is most likely the
rate-determining step. In homogeneous catalysis using metal
halides, the addition of a Lewis acid such as Co2+ ions to the
reaction mixture enhanced H2 production activity, which can
be simply explained by cooperative activation of Lewis-basic
AB with the assistance of the electron-deficient Lewis-acidic
Co2+ ions.[19] Thus, we predict that the neighboring Lewis-
acidic Ce sites (NiÀOÀCe and FeÀOÀCe) also have a positive
effect on the enhancement of catalytic activity.
Materials
CeO2 (JRC-CEO-1) was supplied from the Japan Catalysis Society.
FeSO4·7H2O, NiCl2·6H2O, and ZrO2 were purchased from Nacalai
Tesque. Ammonia borane (NH3–BH3, AB) and dimethylamine–
borane ((CH3)2NHBH3; DMAB) were obtained from Aldrich Chemical
Co. All commercially available compounds were used as received.
The ordinary distilled water was used as reaction solvent.
Preparation of catalyst
CeO2 (0.5 g) was mixed with a 100 mL volume of aqueous solution
containing FeSO4·7H2O (0.05 g, 0.18 mmol), NiCl2·6H2O (0.043 g,
0.18 mmol) and stirred at room temperature for 1 h. The suspen-
sion was evaporated under vacuum, and the obtained powder was
dried overnight, giving FeNi/CeO2. In this way, the FeNi/CeO2 sam-
ples with different Fe/Ni ratios (Fe:Ni=1:0, 2:1, 1:1, 1:2, 1:3, and
0:1) were prepared. FeNi/TiO2, FeNi/ZrO2, FeNi/Al2O3, FeNi/SiO2,
FeNi/SBA-15, and FeNi/MgO were also synthesized according to
this procedure.
The effect of a reducing reagent in the preparation of FeNi
NPs on the catalytic activity was also evidenced by the XAFS
analysis. The Ni K-edge XANES and FT EXAFS spectra of FeNi/
CeO2 prepared by reduction with molecular hydrogen at 773 K
are similar to those of Ni foil but differ from those of NiO and
NiFe2O4 (Figure 9). Fe K-edge XAFS spectra also confirm the
formation of FeÀNi bonds in the metallic form (Figure 10).
Such an observation in H2-treated FeNi/CeO2 is consistent with
the results of NaBH4-treated FeNi/ZrO2, as described earlier,
verifying the crystalline nature of the particles. Thus, we con-
clude that the amorphous character is essential to attain high
activity of FeNi NPs in the dehydrogenation of AB, because of
the exposure of active NiÀOÀCe and FeÀOÀCe sites for the cat-
alytic reaction compared to its crystalline counterparts. Similar
phenomena have been reported for Fe NP-catalyzed dehydro-
genation of AB, in which in situ synthesized amorphous Fe NPs
composed of zero-valent Fe showed unexpectedly higher ac-
tivity in the hydrogenation of AB than as-synthesized a-Fe
crystallites.[8c]
Catalytic study using in situ synthesized FeNi alloy catalyst
In a typical experiment, the FeNi/CeO2 catalyst (0.02 g) was placed
into a Schlenk-type reaction vessel (30 mL) connected with gas bu-
rette. After the purging with Ar for three times, a 10 mL volume of
aqueous solution of NaBH4 (0.015 mmol) was added and gently
stirred for 1 h. The evolution of gas was monitored by using a gas
burette. After the reduction was completed, a 1 mL volume of
aqueous solution of AB (1.92 mmol) was further added into the re-
action vessel and reacted at 303 K to study the catalytic activity of
the as-synthesized catalyst. The reaction was started when the AB
solution was added to the reaction vessel.
Characterization
Conclusions
Powder X-ray diffraction patterns were recorded by using a Rigaku
Ultima IV diffractometer with CuKa radiation (l=1.5406 ). BET sur-
face area measurements were performed by using a BEL–SORP
max (Bel Japan, Inc.) instrument at 77 K. The sample was degassed
in vacuum at 353 K for 24 h prior to data collection. Inductively
coupled plasma optical emission spectrometry measurements were
performed using a Nippon Jarrell-Ash ICAP-575 Mark II instrument.
SEM–EDX measurement was performed by HITACHI SU8220 FESEM
equipped with XFLASH5060FQ detector. Fe K-edge and Ni K-edge
XAFS spectra were recorded at RT in fluorescence mode at the BL-
9A facility at the Photon Factory in the National Laboratory for
High-Energy Physics, Tsukuba (2012G126). A Si(111) double crystal
The synthesis and characterization of FeNi/CeO2 and an evalua-
tion of its catalytic activity in the catalytic dehydrogenation of
ammonia–borane (NH3–BH3) are presented. The impregnated
iron(II) and nickel(II) ions on CeO2 were reduced with NaBH4
prior to catalytic dehydrogenation of ammonia–borane. CeO2
was proven to be an appropriate catalyst support to optimize
catalytic activity. Characterization by several physicochemical
methods revealed that the highly dispersed FeNi NPs were sta-
bilized on the surface of CeO2 by strong interactions. In con-
trast, FeNi/ZrO2, which exhibited inferior activity to FeNi/CeO2,
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