Influence of the Water/Titanium Alkoxide Ratio on the Morphology and Catalytic Activity of Titania–Nickel Composite Particles for the Hydrolysis of Ammonia Borane

Article abstract of DOI:10.1002/open.201800116

This work reports the influence of the water/titanium alkoxide ratio during the preparation of titania–nickel composite particles on their morphology and catalytic activity toward the hydrolysis of ammonia borane. The titania–nickel composite particle catalysts were fabricated by using a sol-gel method, followed by an activation process in aqueous solution containing sodium borohydride and ammonia borane. From the scanning electron microscopy images and pore-size distributions calculated from nitrogen sorption data, the particle dispersion was significantly enhanced at ratios above 6000, and increased with increasing water/titanium alkoxide ratio. Stoichiometric amounts of hydrogen were evolved in the presence of all of the prepared titania–nickel composite particle catalysts. The particle dispersion influenced the hydrogen evolution rate from aqueous ammonia borane solution, and the samples with the most highly dispersed particles showed the highest hydrogen evolution rate. The most active catalyst showed an apparent activation energy comparable to that of other reported catalysts and high cycle ability for the hydrolysis of ammonia borane.

Full text of DOI:10.1002/open.201800116

DOI: 10.1002/open.201800116
Influence of the Water/Titanium Alkoxide Ratio on the
Morphology and Catalytic Activity of Titania–Nickel
Composite Particles for the Hydrolysis of Ammonia Borane
Tetsuo Umegaki,*[a] Yoshifumi Yamamoto,[a] Qiang Xu,[b] and Yoshiyuki Kojima[a]
This work reports the influence of the water/titanium alkoxide
ratio during the preparation of titania–nickel composite parti-
cles on their morphology and catalytic activity toward the hy-
drolysis of ammonia borane. The titania–nickel composite par-
ticle catalysts were fabricated by using a sol-gel method, fol-
lowed by an activation process in aqueous solution containing
sodium borohydride and ammonia borane. From the scanning
electron microscopy images and pore-size distributions calcu-
lated from nitrogen sorption data, the particle dispersion was
significantly enhanced at ratios above 6000, and increased
with increasing water/titanium alkoxide ratio. Stoichiometric
amounts of hydrogen were evolved in the presence of all of
the prepared titania–nickel composite particle catalysts. The
particle dispersion influenced the hydrogen evolution rate
from aqueous ammonia borane solution, and the samples with
the most highly dispersed particles showed the highest hydro-
gen evolution rate. The most active catalyst showed an appar-
ent activation energy comparable to that of other reported
catalysts and high cycle ability for the hydrolysis of ammonia
borane.
1. Introduction
Secure storage and effective release of hydrogen remains one
of the major technological barriers for the widespread applica-
tion of hydrogen energy.[1,2] Currently, considerable efforts are
being devoted to the search for suitable hydrogen storage ma-
terials including metal hydrides, sorbent materials, and chemi-
cal hydrides.[3] Among them, ammonia borane (NH3BH3) has at-
tracted much attention as a hydrogen carrier for portable hy-
drogen storage applications, owing to its high gravimetric hy-
drogen density (19.6 wt%), high stability and solubility under
ambient conditions, environmentally friendliness, and favorable
kinetics for hydrogen release.[4–6] In the presence of suitable
catalysts, ammonia borane releases hydrogen via hydrolysis
under ambient conditions. Therefore, the development of cata-
lysts able to boost the kinetics of hydrogen generation
through ammonia borane hydrolysis is highly relevant for both
academia and industry.
The hydrolysis of ammonia borane in the presence of a cata-
lyst has been well investigated. Noble-metal-based catalysts,
such as those incorporating Au, Ru, Pt, Rh, and Ir, have shown
highly efficient hydrogen generation from aqueous ammonia
borane solution under mild conditions; however, their low
abundance and high price hamper their industrial applica-
tion.[7–11] Meanwhile, low-cost and resourceful catalysts based
on transition metals (Fe, Co, Ni, and Cu) have also been stud-
ied.[12–23] The catalytic performance of metal nanoparticles and
composite particles containing metals is highly dependent on
the dispersion of the active metals and/or composite parti-
cles.[18,24] For example, our research group found that the Si+
Ni content influences the dispersion of silica–nickel composite
particles and their catalytic activity for the hydrolytic dehydro-
genation of ammonia borane.[18] The results also indicated that
the ratio of water to silicon alkoxide influences the dispersion
and activity of the catalyst.
In the present study, we have investigated the influence of
the preparation conditions on the dispersion and catalytic ac-
tivity of titania–nickel composite particle catalysts toward hy-
drogen evolution from aqueous ammonia borane. We have
previously reported that titania–nickel composite catalysts
show higher activity for the hydrolysis reaction than silica–
nickel composite catalysts.[17] To improve their dispersion, we
focused on the control of the titanium alkoxide sol-gel reaction
equilibrium, and then investigated the influence of the water/
titanium alkoxide ratio on the dispersion of the titania–nickel
composite particle catalysts and their activity for the hydrolysis
of ammonia borane.
[a] Dr. T. Umegaki, Y. Yamamoto, Prof. Dr. Y. Kojima
Department of Materials & Applied Chemistry
College of Science & Engineering, Nihon University
1-8-14, Kanda-Surugadai, Chiyoda-Ku, Tokyo 101-8308 (Japan)
E-mail: umegaki.tetsuo@nihon-u.ac.jp
[b] Prof. Dr. Q. Xu
National Institute of Advanced Industrial Science and Technology (AIST)
1-8-31, Midorigaoka, Ikeda, Osaka (Japan)
The ORCID identification number(s) for the author(s) of this article can
be found under:
https://doi.org/10.1002/open.201800116.
ꢀ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are
made.
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2. Results and Discussion
The morphology of the titania–nickel composite particle cata-
lysts prepared at varying water/titanium alkoxide molar ratios
was studied by TEM imaging (Figure 1). The sample prepared
Figure 2. Pore-size distributions of the titania–nickel composite particle cata-
lysts prepared at water/titanium alkoxide ratios of a) 3000, b) 6000, c) 9000,
and d) 12000.
droxide and its subsequent condensation (Figure 3). Conven-
tionally, particles are prepared at water/titanium alkoxide ratios
of the order of ten through sol-gel-based methods.[25–28] On
Figure 1. TEM images of the titania–nickel composite particle catalysts pre-
pared at water/titanium alkoxide ratios of a) 3000, b) 6000, c) 9000, and
d) 12000, after the activation process.
with a water/titanium alkoxide molar ratio of 3000 consisted of
1–3 mm aggregates, including fine primary particles with a di-
ameter of up to 100 nm (Figure 1a). The fine primary particles
were found to be uniformly dispersed at ratios of over 6000,
where their dispersion improved with the increasing water/tita-
nium alkoxide molar ratio (Figure 1b–d). Such enhanced dis-
persion with the increasing ratio was also confirmed by nitro-
gen sorption measurements. All the physicochemical proper-
ties (specific surface area, pore volume, and average pore di-
ameter) were significantly enhanced at ratios over 6000, also
increasing with the increasing ratio (Table 1). The results of
their pore-size distributions also reflect this tendency. The di-
ameter of the mesopores and macropores of the samples sig-
nificantly increased at water/titanium alkoxide molar ratios of
6000, further increasing with the water/titanium alkoxide
Figure 3. Scheme of the preparation process based on a sol-gel reaction.
the other hand, the water/titanium alkoxide molar ratios in the
present study were 102–103 times higher than those employed
under conventional preparation conditions. These results indi-
cate that the condensation reaction equilibrium is suppressed
with significantly large amounts of water relative to the titani-
um alkoxide content. As shown in high resolution TEM images
(Figure 4), the morphology of the catalysts did not significantly
change before and after the activation process.
The valence state of the active nickel species in the titania–
nickel composite particle catalysts was characterized by DRUV/
Vis absorption spectroscopy before and after the activation
molar ratio (Figure 2). The preparation process of the titania– process in aqueous solution containing sodium borohydride
nickel composite particles is based on a sol-gel reaction, which
consists of the hydrolysis of titanium alkoxide into titanium hy-
and ammonia borane (Figure 5). In the spectra of the precur-
sors, the observed absorption bands at 550–650 nm can be at-
!
3
3
tributed to the electronic transition T1g(F) A2g(F) for Ni2+ [29]
,
indicating that all the precursors contain NiII with octahedral
coordination.
Table 1. Physicochemical properties of the titania–nickel composite parti-
Compared to the spectra of each precursor, the intensity of
the peaks in the spectra after the activation process was negli-
gible. The results indicate that active metallic nickel species
were obtained in all samples through the activation process.
Figure 6 shows the XPS spectra of the sample prepared at a
water/titanium alkoxide ratio of 12000. The spectrum of the ti-
tania–nickel composite particle precursor exhibits a Ni2p3/2
cle catalysts prepared at different water/titanium alkoxide ratios.
[a]
[b]
H2O/titanium alkoxide
SBET [m2 gÀ1
]
V [cm3 gÀ1
]
D [nm][b]
3000
6000
9000
12000
56.7
79.6
71.0
83.5
0.0755
0.2448
0.2847
0.3182
5.31
12.05
16.47
15.32
[a] Brunauer–Emmett–Teller (BET) surface area. [b] Calculated from the ad-
sorption branch of their nitrogen adsorption-desorption isotherms.
band at 855.6 eV, typical of Ni2+ [22]
On the other hand, Ni2p3/2
.
bands at 856.2 and 852.5 eV are observed in the spectrum of
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Figure 6. XPS spectra of the titania–nickel composite particles prepared at a
water/titanium alkoxide ratio of 12000: a) precursor, b) catalyst, c) catalyst
after Ar+ sputtering for 5 min.
Figure 4. High-resolution TEM images of the titania–nickel composite parti-
cle catalyst prepared at water/titanium alkoxide ratio of a,b) 9000 and
c,d) 12000 before (a,c) and after (b,d) the activation process.
nickel composite particles, and that part of such nickel species
were oxidized after the activation process, probably owing to
exposure to air before XPS measurement. To identify the va-
lence state of nickel species inside the titania–nickel composite
particles, the spectrum of the titania–nickel composite particle
catalyst was obtained after Ar+ sputtering. The spectrum in
Figure 6c exhibits only a Ni2p3/2 band at 852.7 eV, which is typ-
ical of metallic Ni0. The results indicate that the nickel species
inside the titania–nickel composite particles do not oxidize in
air, but maintain their metallic state.
The evolution of hydrogen from aqueous ammonia borane
solution in the presence of the composite particle catalysts
was evaluated. Figure 7 shows the time course of the hydro-
gen evolution from aqueous solution in the presence of the
catalysts prepared at different water/titanium alkoxide ratios. A
hydrogen evolution of 59.0, 60.0, 59.0, and 60.0 mL from the
aqueous ammonia borane solution was completed within ap-
Figure 5. DRUV/Vis spectra of the titania–nickel composite particles pre-
pared at water/titanium alkoxide ratios of a,e) 3000, b,f) 6000, c,g) 9000,
and d,h) 12000 before (a–d) and after (e–h) the activation process in aque-
ous solution containing sodium borohydride and ammonia borane.
the titania–nickel composite particle catalyst, which are typical
of Ni2+ and metallic Ni0, respectively.[22] The results indicate
Figure 7. Hydrogen evolution from the hydrolysis of ammonia borane cata-
lyzed by the titania–nickel composite particle catalysts prepared at water/ti-
that some nickel species exist on the surface of the titania– tanium alkoxide ratios of a) 3000, b) 6000, c) 9000, and d) 12000.
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proximately 20, 14, 11, and 10 min in the presence of the cata-
lysts prepared at water/titanium alkoxide ratios of 3000, 6000,
9000, and 12000, respectively. Hydrogen is produced from
aqueous ammonia borane via Reaction (1):
À
NH3BH3 2 H2O ! NH4
BO2
3 H2
1
Under the present reaction conditions, approximately 59 mL
of hydrogen (24.0”10À4 mol) was generated through this reac-
tion. The molar ratio of the hydrolytically generated hydrogen
to the initial ammonia borane in the presence of the titania–
nickel composite particle catalyst was determined as 3.0. The
results indicate that stoichiometric amounts of hydrogen were
evolved in the presence of the different titania–nickel compo-
site particle catalysts, because the reducibility of the active
nickel species is very similar in all the catalysts. On the other
hand, the hydrogen evolution rate was found to depend on
the catalyst; the rate in the presence of the titania–nickel com-
posite particle catalysts prepared at water/titanium alkoxide
ratios of 3000, 6000, 9000, and 12000 was 3.33, 5.00, 6.00, and
6.67 mLminÀ1, respectively. These results, in addition to those
shown in Figures 1 and 2 and Table 1, indicate that the catalyst
with the most highly dispersed primary particles shows the
highest hydrogen evolution rate, probably because of a higher
diffusion rate of the reactants into the active sites.
Figure 8. Arrhenius plots for the hydrolysis of ammonia borane catalyzed by
the titania–nickel composite particle catalyst prepared at a water/titanium
alkoxide ratio of 12000.
Figure 9a shows the time course at the 1st, 5th, 9th, 11th,
and 15th cycle for the hydrogen evolution from aqueous am-
monia borane solution in air catalyzed by the titania–nickel
composite particle catalyst prepared at a water/titanium alkox-
The catalytic hydrolysis of ammonia borane was further eval-
uated with the titania–nickel composite particle catalyst pre-
pared at a water/titanium alkoxide ratio of 12000 at various
temperatures in the range of 298–343 K, so as to determine
the apparent activation energy [Eq. (2)].
À3d NH3BH3Š=dt d H2Š=dt
k
2
where k is the reaction rate, [NH3BH3] and [H2] are the amount
of NH3BH3 and H2, respectively. Under our experimental condi-
tions, the reaction rate, k, is constant at each given tempera-
ture, implying zero order kinetics for the ammonia borane hy-
drolysis reaction. This suggests that the hydrogen generation
rate is controlled by surface reactions. The reaction rate equa-
tion can be written as follows [Eq. (3)]:
k
k0 exp ÀEa=RT
3
where k0 is the reaction constant (mLminÀ1), Ea the activation
energy for the reaction, R the gas constant, and T is the reac-
tion temperature. Using Equation (3), the value of the rate con-
stant k at different temperatures for the hydrolysis of ammonia
borane was calculated from the linear segment of the time
course of hydrogen evolution in the presence of the catalyst
prepared at a water/titanium alkoxide ratio of 12000 at differ-
ent temperatures in the range of 298–343 K. The Arrhenius
plots are shown in Figure 8. They were also used to calculate
the activation energy parameter (Ea =37.5 kJmolÀ1) for the cat-
alyst. The obtained activation energy is comparable to that of
other reported nickel-based catalysts.[21,30,31]
Figure 9. a) Time course for the evolution of hydrogen from aqueous ammo-
nia borane solution catalyzed by the titania--nickel composite particle cata-
lyst prepared at a water/titanium alkoxide ratio of 12000 for the 1st, 5th,
9th, 11th, and 15th cycle and b) the hydrogen evolution rate at each cycle.
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cursor, sodium borohydride, and ammonia borane, and the activa-
tion process was carried out at 343 K in air.
ide of 12000. The amount of hydrogen evolution does not
depend on the cycle, as approximately 60 mL of hydrogen was
evolved in all cycles. The results indicate that stoichiometric
amounts of hydrogen were evolved. On the other hand, the
evolution rate was found to depend on the cycle number. The
hydrogen evolution rate dropped significantly from the 5th to
the 9th cycle, to then decrease further in a lesser way. Fig-
ure 9b plots the hydrogen evolution rate against the cycle
number. Up to the 4th cycle, the hydrogen evolution rate was
stable at around 6.5 mLminÀ1, whereas it decreased from the
5th to the 9th cycle. Then, the rate was maintained at approxi-
mately 4.0 mLminÀ1 up to the 15th cycle. From EDX measure-
ments of the sample before activation process and after the
10th cycle, the nickel content included in the sample is 2.36
and 1.67 mol%, respectively. In addition, the peak assigned as
NiII was not observed in the UV/Vis spectrum of the catalyst
after 10th cycle. From these results, the active metallic nickel
species remained in the catalyst, whereas a decrease in the hy-
drogen evolution rate is probably attributable to dissolution
and/or oxidation of some of the active metallic nickel species
on the surface of the titania–nickel composite particles.
Characterization
The morphology of the titania–nickel composite particle catalysts
was observed on a Hitachi S-450 scanning electron microscope op-
erating at an acceleration voltage of 15 kV. The physicochemical
properties of the composite particles were evaluated by nitrogen
sorption isotherms at 77 K on a Micromeritics Model ASAP 2010MC
analyzer. Diffuse reflectance ultraviolet and visible (DRUV/Vis) spec-
tra were recorded over the range of 200–800 nm to identify the va-
lence state of the nickel species in the samples on a UV-3600 (Shi-
madzu Co. Ltd.). UV/Vis spectrophotometer using barium sulfate as
the standard. X-ray photoelectron spectra were acquired on an
ESCA-3400 spectrometer (Shimadzu Co. Ltd.) equipped with a
MgKa X-ray excitation source (1253.6 eV) operating at 10 kV and
10 mA. The binding energies (BEs) are referenced to the C1s peak
at 285.0 eV.
Experimental Procedure for the Hydrolysis of Ammonia
Borane
An aqueous ammonia borane solution (0.16m, 5 mL) was added to
the catalyst suspension after the activation process described
above, and the evolution of gas was monitored at 298 K using the
gas burette for the evaluation of the catalytic activity. The hydro-
gen evolution reaction was carried out at 298, 323 and 343 K for
the assessment of the activation energy. For the cyclability tests,
additional equivalents of said aqueous ammonia borane solution
(0.16m, 5 mL) were added to the reaction flask once the previous
hydrogen evolution reaction was completed. Such cycles were car-
ried out fifteen times in air.
3. Conclusions
The present work describes the influence of the water/titanium
alkoxide ratio during the preparation of titania–nickel compo-
site particle catalysts on their morphology and activity toward
the hydrolysis of ammonia borane. From their TEM images and
pore-size distributions, the dispersion of the particles was en-
hanced at ratios above 6000, increasing with increasing water/
titanium alkoxide ratio. Stoichiometric amounts of hydrogen
were evolved in the presence of all the prepared titania–nickel
composite particle catalyst. The dispersion of the particles was
found to influence the hydrogen evolution rate from the aque-
ous ammonia borane, and the sample with the most highly
dispersed particles showed the highest hydrogen evolution
rate. The most active catalyst showed a comparable apparent
activation energy that other reported catalysts and high cycla-
bility for 15 cycles.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: ammonia borane · composite particles · hydrogen
evolution · titania–nickel · water/titanium alkoxide ratio
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Received: June 20, 2018
Revised manuscript received: July 18, 2018
Version of record online August 15, 2018
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