Rhodium(0) nanoparticles supported on nanotitania as highly active catalyst in hydrogen generation from the hydrolysis of ammonia borane

Article abstract of DOI:10.1039/c4ra00469h

Rhodium(0) nanoparticles supported on the surface of titanium dioxide (Rh(0)@TiO₂) were in situ generated from the reduction of rhodium(iii) ions impregnated on nanotitania during the hydrolysis of ammonia borane. They were isolated from the reaction solution by centrifugation and characterized by a combination of advanced analytical techniques. The results show that (i) highly dispersed rhodium(0) nanoparticles with sizes in the range 1.3-3.8 nm were formed on the surface of titanium dioxide, (ii) Rh(0)@TiO ₂ shows high catalytic activity in hydrogen generation from the hydrolysis of ammonia borane with a turnover frequency value up to 260 min ^-1 at 25.0 ± 0.1°C, (iii) the results of kinetic studies on the hydrogen generation from the hydrolysis of ammonia borane were also reported including the activation energy of 65.5 ± 2 kJ mol^-1 for this reaction. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra00469h

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Rhodium(0) nanoparticles supported on
nanotitania as highly active catalyst in hydrogen
generation from the hydrolysis of ammonia borane
Cite this: RSC Adv., 2014, 4, 13742
a
b
b
a
Serdar Akbayrak, Serap Gençturk, ˙I zzet Morkan and Saim O¨ zkar*
¨
2
Rhodium(0) nanoparticles supported on the surface of titanium dioxide (Rh(0)@TiO ) were in situ generated
from the reduction of rhodium(III) ions impregnated on nanotitania during the hydrolysis of ammonia
borane. They were isolated from the reaction solution by centrifugation and characterized by a
combination of advanced analytical techniques. The results show that (i) highly dispersed rhodium(0)
nanoparticles with sizes in the range 1.3–3.8 nm were formed on the surface of titanium dioxide, (ii)
Received 16th January 2014
Accepted 31st January 2014
2
Rh(0)@TiO shows high catalytic activity in hydrogen generation from the hydrolysis of ammonia borane
ꢀ
1
ꢂ
with a turnover frequency value up to 260 min at 25.0 ꢁ 0.1 C, (iii) the results of kinetic studies on the
hydrogen generation from the hydrolysis of ammonia borane were also reported including the activation
energy of 65.5 ꢁ 2 kJ molꢀ for this reaction.
DOI: 10.1039/c4ra00469h
1
www.rsc.org/advances
years. Metal nanoparticles exhibit much higher catalytic activity
compared to the bulk metal due to the large fraction of atoms
Introduction
Ammonia-borane (NH BH , AB) has been regarded as one of the on their surface. However, metal nanoparticles tend to aggre-
3
3
9,10
leading candidates for hydrogen storage materials for on-board gate into clumps causing a decrease in catalytic activity.
hydrogen applications due to its high hydrogen storage capacity Titanium dioxide with a large surface area ranging from 10 to
19.6 wt%), non-toxicity and high stability under ambient 300 m g can be used as a support to prevent the aggregation
conditions. Hydrogen stored in the AB complex can be released of metal nanoparticles. Titanium dioxide is one of the suitable
by either thermolysis or solvolysis. Since high temperature is catalysts for environmental applications because of its non-
2
ꢀ1
(
1
2
3
4
required for the former, the latter reaction is highly advanta- toxicity, strong oxidizing power and the high stability against
geous for hydrogen generation from AB. However, hydrogen corrosion. Titanium dioxide has also been extensively studied
11
stored in AB can be released at an appreciable rate via hydrolysis as a semiconductor material due to its wide band gap and low
5,6
12
only in the presence of a suitable catalyst (eqn (1)). Therefore, cost.
the development of efficient and stable catalysts under
Herein we report the in situ generation, characterization, and
moderate conditions is vital for practical applications.
catalytic use of rhodium(0) nanoparticles supported on tita-
nium dioxide with particle sizes of about 100 nm, hereaer
catalyst
H
3
NBH
3
ðaqÞ þ 2H
2
OðlÞ ƒƒƒ ƒƒ ! NH
4
þðaqÞþBO
2
ꢀðaqÞ þ 3H
ðgÞ
2
referred to as Rh(0)@TiO
1) the surface of titania particles were reduced by ammonia
borane forming the Rh(0)@TiO without using any additional
reducing agents during the hydrolysis of ammonia borane. The
use of in situ generation of Rh(0)@TiO provides the opportu-
2
. Rhodium(III) ions impregnated on
(
2
A number of catalysts have been tested in hydrogen gener-
2
ation from the hydrolysis of AB, including transition metal
nanoparticles such as platinum, rhodium, ruthenium, palla-
nity to avoid laborious catalyst preparation steps. Rh(0)@TiO2
was isolated from the reaction solution and characterized using
ICP-OES, XRD, TEM, SEM-EDS, XPS and N2 adsorption–
desorption techniques. Our report also includes the following
major ndings: (i) formation of highly dispersed rhodium(0)
nanoparticles on the surface of titanium dioxide with particle
sizes in the range 1.3–3.8 nm. (ii) Remarkable catalytic activity
7,8
dium, cobalt and nickel. Although rhodium, platinum and
ruthenium metal nanoparticles have high prices and limited
abundance, they are superior to the non-noble metal nano-
particles due to their long life-time and high activity in the
hydrolysis of ammonia borane. Therefore the use of noble metal
nanoparticles as catalysts has been intensively studied in recent
2
of Rh(0)@TiO in hydrogen generation from the hydrolysis of
ꢀ
1
ꢂ
AB with a turnover frequency of 260 min at 25 ꢁ 0.1 C. (iii)
a
Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey.
Reusability of Rh(0)@TiO providing the complete hydrolysis of
2
E-mail: sozkar@metu.edu.tr; Fax: +90 312 210 3200; Tel: +90 312 210 3212
b
˙
AB generating 3 mole H per mole of AB even in the h use of
2
Department of Chemistry, Abant Izzet Baysal University, 14280 Bolu, Turkey. E-mail:
morkan_i@ibu.edu.tr; Fax: +90 374 253 46 42; Tel: +90 374 254 1000-1246
the catalyst.
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the ask and the reaction medium was stirred at 1000 rpm.
Aer adding ammonia borane, rhodium(0) nanoparticles were
formed and the catalytic hydrolysis of AB started immediately.
Experimental section
Materials
Rhodium(III) chloride trihydrate (RhCl
3
$3H
2
O), titanium(IV) The volume of hydrogen gas evolved was measured by recording
oxide (anatase), and ammonia-borane (AB, 97%) were the displacement of the water level every 30 s at a constant
purchased from Aldrich. Deionized water was distilled by a atmospheric pressure of 693 Torr. The reaction was stopped
water purication system (Milli-Q System). All glassware and when no more hydrogen evolution was observed. In each
Teon-coated magnetic stir bars were cleaned with acetone, experiment, the resulting solutions were ltered and the
1
1
followed by copious rinsing with distilled water before drying in ltrates were analyzed by B NMR and conversion of AB to the
ꢂ
an oven at 150 C.
metaborate anion was conrmed by comparing the intensity of
signals in the B NMR spectra of the ltrates.
1
1
Characterization
Determination of activation energy for hydrolysis of AB
2
The rhodium content of the Rh(0)@TiO sample was deter-
mined by Inductively Coupled Plasma Optical Emission Spec-
catalyzed by Rh(0)@TiO
2
troscopy (ICP-OES, Leeman-Direct Reading Echelle) aer the
In a typical experiment, the hydrolysis reaction was performed
starting with 10 mL of 100 mM (31.8 mg) AB solution and 20 mg
sample was completely dissolved in a mixture of HNO –HCl
3
3
+
(1 : 3 ratio). Transmission electron microscopy (TEM) was per-
Rh @TiO
2
(0.24 wt% rhodium, [Rh] ¼ 0.046 mM) at various
formed on a JEM-2100F (JEOL) microscope operating at 200 kV.
A small amount of powder sample was placed on the holey
carbon grid of the transmission electron microscope. Samples
were examined at magnication between 100 and 400 K. Scan-
ning electron microscope (SEM) images were taken using a
ꢂ
temperatures (25, 30, 35, 40, 45 C) in order to obtain the acti-
vation energy.
Reusability of Rh(0)@TiO in hydrolysis of AB
2
JEOL JSM-5310LV at 15 kV and 33 Pa in a high-vacuum mode Aer the complete hydrolysis of AB started with 10 mL of 100
3
+
without metal coating on the carbon support. The X-ray mM AB (31.8 mg H NBH ), and 100 mg Rh @TiO (0.24 wt%
3
3
2
ꢂ
photoelectron spectroscopy (XPS) analysis was performed on a rhodium, [Rh] ¼ 0.233 mM) at 25 ꢁ 0.1 C, the catalyst was
ꢂ
Physical Electronics 5800 spectrometer equipped with a hemi- isolated as a powder by centrifugation and dried at 120 C in an
spherical analyzer and using monochromatic Al Ka radiation of oven aer washing with 20 mL of water. The isolated samples of
1
486.6 eV, the X-ray tube working at 15 kV, 350 W and pass Rh(0)@TiO were weighed and redispersed in 10 mL solution of
2
1
1
ꢂ
energy of 23.5 keV. B NMR spectra were recorded on a Bruker 100 mM AB for a subsequent run of hydrolysis at 25 ꢁ 0.1 C.
Avance DPX 400 with an operating frequency of 128.15 MHz
1
1
for B.
Determination of the catalytic lifetime of Rh(0)@TiO in the
2
hydrolysis of AB
Preparation of rhodium(III) ions impregnated on TiO
2
The catalytic lifetime of Rh(0)@TiO in the hydrolysis of AB was
determined by measuring the total turnover number (TTO).
Such a lifetime experiment was started with a 50 mL solution
2
TiO
mg) in 100 mL H
temperature for 72 h and then the Rh @TiO
2
(1000 mg) was added to a solution of RhCl
3 2
$3H O (53.97
2
O in a beaker. This slurry was stirred at room
3
+
2
sample was iso-
containing 0.046 mM Rh(0)@TiO
and 30 mM AB at 25.0 ꢁ
2
lated by centrifugation and washed with 100 mL of distilled
ꢂ
0
.1 C. When all the ammonia-borane present in the solution
ꢂ
water and the remnant was dried at 120 C for 12 h in the oven.
was completely hydrolyzed, more AB was added and the reac-
tion was continued in this way until no hydrogen gas evolution
was observed.
In situ formation of rhodium(0) nanoparticles supported on
TiO and concomitant catalytic hydrolysis of AB
2
2 2
Rhodium(0) nanoparticles supported on TiO (Rh(0)@TiO )
were in situ generated from the reduction of Rh @TiO during
2
Results and discussion
3
+
the catalytic hydrolysis of AB. Before starting the catalyst Rhodium(0) nanoparticles supported on titanium dioxide were
3
+
formation and concomitant catalytic hydrolysis of AB, a jack- in situ generated from the reduction of Rh @TiO during the
2
eted reaction ask (20 mL) containing a Teon-coated stir bar catalytic hydrolysis of AB. First, rhodium(III) ions were impreg-
was placed on a magnetic stirrer (Heidolph MR-301) and ther- nated on titania with particle sizes of 100 nm from an aqueous
ꢂ
3+
mostated to 25.0 ꢁ 0.1 C by circulating water through its jacket solution of rhodium(III) chloride yielding Rh @TiO
from a constant temperature bath. Then, a graduated glass tube reduced by AB at room temperature. Both reduction of rho-
60 cm in height and 3.0 cm in diameter) lled with water was dium(III) to rhodium(0) and hydrogen release from the hydro-
connected to the reaction ask to measure the volume of the lysis of AB occur concomitantly when the AB solution is added
2
and then
(
3
+
hydrogen gas to be evolved from the reaction. Next, 20 mg to a suspension of Rh @TiO . The progress of the hydrolysis of
powder of Rh @TiO (0.24 wt% Rh) was dispersed in 10 mL AB was followed by monitoring the change in hydrogen pressure
2
3
+
2
distilled water in the reaction ask thermostated at 25.0 ꢁ which was then converted into the equivalent H
2
per mole of AB,
: AB stoichiometry (eqn (1)).
ꢂ
0
.1 C. Then, 31.8 mg AB (1.0 mmol H
3
N$BH
3
) was added into using the known 3 : 1 H
2
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Characterization of rhodium(0) nanoparticles supported on
TiO
2
Rhodium(0) nanoparticles supported on titania (Rh(0)@TiO
2
),
in situ formed during the hydrolysis of AB, could be isolated
from the reaction solution as a powder by centrifugation and
characterized by ICP-OES, XRD, SEM-EDS, TEM, XPS and N
adsorption–desorption techniques. Rhodium content of Rh(0)
TiO was determined by ICP-OES and found as 0.24 wt% Rh
2
@
2
loading on the titania surface. The N adsorption–desorption
2
2
ꢀ1
analysis gave the surface area of titania as 10.71 m g . Since
the rhodium content of Rh(0)@TiO was very low, it is
difficult to understand the existence of rhodium(0) nano-
particles on the surface of titania by N adsorption–desorption
analysis. Comparison of the XRD patterns of TiO
and Rh(0)@TiO with a rhodium loading of 0.24 wt% Rh, given
2
2
3
+
2
, Rh @TiO
2
2
in Fig. 1a–c, respectively, clearly shows that there is no change
in the characteristic diffraction peaks of TiO2 (PDF card 21-
1272). This observation indicates that the host material remains
intact aer impregnation and reduction of rhodium(III) ions;
there is no noticeable alteration in the framework lattice or
change in the crystallinity. There is no observable peak attrib-
utable to rhodium nanoparticles in Fig. 1b and c, probably
Fig. 2 (a) SEM image and (b) SEM-EDS spectrum of Rh(0)@TiO
2
with a
0
.24 wt% Rh loading.
2
because of the low rhodium loading on the TiO .
The SEM image of Rh(0)@TiO with a rhodium loading of
.24 wt% in Fig. 2a shows the presence of nearly monodispersed
2
studied by XPS. The survey-scan XPS spectrum of Rh(0)@TiO2
with a rhodium loading of 0.24 wt%. (Fig. 4) shows that
0
titania nanoparticles of 100 nm size. The SEM-EDS spectrum of
Rh(0)@TiO with a rhodium loading of 0.24 wt% in Fig. 2b
indicates the presence of the framework elements of TiO
Ti, O). Due to the low rhodium loading on TiO , the presence of
rhodium nanoparticles could not be observed in the SEM
analysis.
rhodium is the only element detected in addition to the TiO
framework elements (Ti, O). High resolution Rh 3d XPS of a
Rh(0)@TiO sample given in the inset of Fig. 4 shows two
prominent bands at 305.8 eV and 311.4 eV which can readily be
2
2
2
2
(
2
13,14
assigned to rhodium(0) 3d5/2 and 3d3/2, respectively.
The
bands at 309.7 eV and 313.8 eV (colored green in the inset of
Fig. 4), might be attributed to rhodium oxides, which might be
Fig. 3 shows the TEM images of Rh(0)@TiO with a rhodium
2
loading of 0.24 wt% taken with different magnications, which
indicate that (i) highly dispersed rhodium(0) nanoparticles are
14
formed during the XPS sampling.
2
formed on the surface of TiO with particle sizes in the range
Catalytic activity of Rh(0)@TiO in hydrolysis of AB
2
1
.3–3.8 nm (mean diameter: 2.8 ꢁ 0.7 nm) and (ii) the
Before starting with the investigation of the catalytic activity of
impregnation of rhodium(III) followed by reduction to
rhodium(0) causes no change in the framework lattice of titania
in agreement with the XRD results.
Rh(0)@TiO in the hydrolysis of AB, a control experiment was
2
performed to check whether titanium dioxide shows any cata-
lytic activity in the hydrolysis of AB at the same temperature
used in this study. In a control experiment starting with 1.0
The composition of Rh(0)@TiO formed in situ during the
2
hydrolysis of AB and the oxidation state of rhodium were also
mmol of AB and 20 mg of powdered TiO
2
(the same amount as
ꢂ
used in catalytic activity tests) in 10 mL of water at 25.0 ꢁ 0.1 C
ꢂ
or 45.0 ꢁ 0.1 C, no hydrogen evolution was observed over 1 h at
both temperatures. This observation indicates that the hydro-
lysis of AB does not occur in the presence of TiO2 in the
temperature range used in this study. However, Rh(0)@TiO is
2
found to be a highly active catalyst in the hydrolysis of AB
2
generating 3.0 equivalents of H gas per mole of AB in the same
temperature range.
Fig. 5a shows the evolution of equivalent hydrogen per mole
of AB versus time plot for the hydrolysis of AB (100 mM) using
Rh(0)@TiO
2
as the catalyst in different rhodium concentrations
ꢂ
at 25.0 ꢁ 0.1 C. The hydrogen generation rate was determined
from the linear portion of each plot. For all tests a complete
3+
hydrogen release (mol H
Fig. 5b shows the plot of hydrogen generation rate versus initial
2
/mol H
3
NBH
3
¼ 3) was observed.
2 2 2
Fig. 1 Powder XRD patterns of (a) TiO , (b) Rh @TiO , (c) Rh(0)@ TiO
with a 0.24 wt% Rh loading.
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2
Fig. 4 X-ray photoelectron (XPS) spectrum of Rh(0)@TiO sample
with a rhodium loading of 0.24 wt% Rh. The inset gives the high
resolution scan and deconvolution of Rh 3d bands.
Fig. 5 (a) mol H
rhodium concentrations in Rh(0)@TiO
mM) at 25.0 ꢁ 0.1 C; (b) the logarithmic plot of hydrogen generation
rate versus the concentration of Rh; ln(rate) ¼ 1.12 ln[Rh] + 4.34.
2
per mol H
3
N$BH
3
versus time graph for various
2
for the hydrolysis of AB (100
ꢂ
2
Fig. 3 TEM image of Rh(0)@TiO with a rhodium loading of 0.24 wt%
@
TiO with a loading of 0.24 wt% Rh. The TOF value of the
2
at (a) 100 nm, (b) 50 nm, (c) 20 nm, (d) 10 nm, (e) histogram of Rh(0)
TiO showing particle size distribution.
ꢀ
1
Rh(0)@TiO catalyst is as high as 260 min (mol H per mol Rh
@
2
2
2
ꢂ
min) in hydrolysis of AB at 25.0 ꢁ 0.1 C. TOF values of the
reported catalysts used in the hydrolysis of AB are listed in Table
concentration of rhodium, both in logarithmic scale, which 1 for comparison. As clearly seen from the comparison of values
gives a straight line with a slope of 1.12 indicating that hydro- listed in Table 1, Rh(0)@TiO provides a remarkable TOF value
2
lysis of AB is rst order with respect to the rhodium concen- in the hydrolysis of AB as compared to the other ruthenium,
tration. The turnover frequency (TOF), mol of H per mol of rhodium and palladium catalysts.
2
rhodium per minute, for hydrogen generation from the hydro-
The catalytic hydrolysis of AB was carried out at various
ꢂ
ꢂ
lysis of AB (100 mM) at 25.0 ꢁ 0.1 C was determined from the temperatures in the range of 25–45 C starting with Rh(0)@TiO2
hydrogen generation rate in the linear portion of the plots given (loading ¼ 0.24 wt% Rh and [Rh] ¼ 0.116 mM) plus 100 mM AB
in Fig. 5a for experiments starting with 100 mM AB plus Rh(0) in 10 mL of water. The rate constants for the hydrogen
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Table 1 Catalytic activity of various reported catalysts used in the hydrolysis of AB
ꢀ1
ꢀ1
Entry
Catalyst
TOF (min
)
E
a
(kJ mol
)
TTO
Ref.
1
2
3
4
5
6
7
8
9
Ru@MWCNT
329
260
200
172
137
114
113
100
92
90.2
75
37.5
35.7
26.3
—
33
26 400
37 350
80 000
51 720
87 000
—
—
—
47 200
36 700
5900
—
15
Rh(0)@TiO
2
65.5
43.6
54
58
44
This study
Laurate-stabilized Rh(0)
Ru(0)PSSA-co-MA
Ru(0)@hap
Ni@Ru
Ru/carbon
Ru/graphene
ZFS Rh(0)
RuNPs@ZK-4
Ru(0)NP/laurate
Pd@Co/graphene
Co35Pd65/C annealed
2.1 wt% RGO@Pd
16
17
18
19
20
21
22
23
24
25
26
27
28
76
11.7
66.9
28
47
—
—
40
21
10
11
12
13
14
15
—
—
—
Rh/g-Al
2
O
3
generation at different temperatures were calculated from the from the reaction solution aer a previous run of hydrolysis of
slope of the linear part of each plot given in Fig. 6a and used for AB. Aer the completion of hydrogen generation from the
ꢀ
1
3+
the calculation of activation energy (E
a
¼ 65 ꢁ 2 kJ mol ) from hydrolysis of AB starting with 0.233 mM Rh @TiO
2
plus 100
ꢂ
the Arrhenius plot in Fig. 6b. The activation energy for hydro- mM AB in 10 mL aqueous solution at 25.0 ꢁ 0.1 C, the catalyst
lysis of AB catalyzed by Rh(0)@TiO
2
is comparable to the liter- was isolated by centrifugation and washed with 10 mL of water.
was redis-
persed in 10 mL solution containing 100 mM AB and a second
2
catalyst was tested in run of hydrolysis was started immediately and continued until
ature values reported for the same reaction using other catalysts Aer washing, the isolated sample of Rh(0)@TiO
2
(see Table 1).
Reusability of the Rh(0)@TiO
successive experiments performed using the catalyst isolated the completion of hydrogen evolution. The hydrogen generation
process was repeated ve times. Aer the h use in the
hydrolysis of AB, Rh(0)@TiO
catalytic activity. The reusability tests reveal that Rh(0)@TiO
still active in the subsequent runs of hydrolysis of AB providing
a release of 3.0 equivalent H per mole of NH BH (Fig. 7).
2
preserved only 7.0% of the initial
2
is
2
3
3
The catalytic activity of the ltrate solution obtained by centri-
fugation of the solid materials aer the rst run of the hydrolysis
was also tested in the hydrolysis of AB (100 mM) under the same
conditions. As shown in Fig. 8, the ltrate solution shows no
catalytic activity in the hydrolysis of AB. This observation supports
the conclusion that there is no leaching of rhodium into the
solution during the hydrolysis and suggests that the rhodium(0)
2
nanoparticles supported on TiO are kinetically competent and a
heterogeneous catalyst in the hydrolysis of ammonia borane.
Fig. 6 (a) The evolution of equivalent hydrogen per mole of AB versus
time plot for the hydrolysis of AB starting with Rh(0)@TiO (0.116 mM Rh) Fig. 7 Percentage of initial catalytic activity of Rh(0)@TiO
2
2
([Rh] ¼
and 100 mM AB at various temperatures. (b) The Arrhenius plot for the 0.233 mM) in successive runs after reuse for the hydrolysis of ammonia
Rh(0)@TiO
2
catalyzed hydrolysis of AB. ln k ¼ ꢀ7883.5(Tꢀ1) + 27.27.
borane (100 mM).
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ammonia borane at room temperature. Highly dispersed
rhodium(0) nanoparticles with particle sizes in the range 1.3–
3.8 nm on titanium dioxide were prepared and characterized by
a combination of advanced analytical techniques. Rh(0)@TiO2
shows high catalytic activity in hydrogen generation from the
hydrolysis of ammonia borane providing a turnover frequency
ꢀ
1
ꢂ
value up to 260 min at 25.0 ꢁ 0.1 C. Rh(0)@TiO
2
is a long-
lived catalyst providing 37 350 turnovers in hydrogen genera-
ꢂ
tion from the hydrolysis of ammonia borane at 25.0 ꢁ 0.1 C.
Rh(0)@TiO is a reusable catalyst as it provides the complete
2
hydrolysis of ammonia borane generating 3 mole H per mole of
AB even in the h use. The results of quantitative kinetic
studies on the hydrogen generation from the hydrolysis of
2
Fig. 8 The evolution of equivalent hydrogen per mole of AB versus
time plot for the hydrolysis of AB (100 mM) starting with Rh(0)@TiO
0.233 mM Rh) (black O), and the filtrate solution obtained by
2
(
centrifugation of the solid materials after the first run, (red O), at room ammonia borane show that the hydrolysis reaction is rst order
temperature.
in rhodium concentration and the activation energy is 65.5 ꢁ 2
ꢀ
1
2
kJ mol for the hydrogen generation catalyzed by Rh(0)@TiO .
High catalytic activity and simple preparation procedures make
The catalytic lifetime of Rh(0)@TiO
2
was determined by Rh(0)@TiO2 a very attractive catalyst for the hydrolysis of
measuring the total turnover number (TTO) in the hydrolysis of ammonia borane.
ammonia borane. A catalyst lifetime experiment was performed
3
+
starting with 20 mg Rh @TiO (rhodium loading ¼ 0.24 wt%
2
Acknowledgements
Rh, and [Rh] ¼ 0.0466 mM) in 100 mL solution of AB at 25.0 ꢁ
ꢂ
0
.1 C. Fig. 9 shows the variation in turnover number (TON) and
Partial support by Turkish Academy of Sciences is gratefully
acknowledged. We would like to thank Emrah Yıldırım, Seçkin
turnover frequency (TOF) in the course of reaction. The TOF
value decreases expectedly as the rhodium(0) nanoparticles
catalysts are deactivated during the lifetime experiment because
of the increasing concentration of metaborate ion. Rhodium(0)
¨
˙
Ozt u¨ rk and Ilker Yıldız for ICP-OES, TEM and XPS analyses,
respectively.
nanoparticles supported on TiO provide 37 350 turnovers over
2
ꢂ
2
0 h in the hydrolysis of AB at 25.0 ꢁ 0.1 C before deactivation.
Notes and references
2
As shown in Table 1, the TTO value of Rh(0)@TiO for the
hydrolysis of ammonia borane is comparable to the literature
values reported for the same reaction using other catalysts.
1 T. Umegaki, J.-M. Yan, X.-B. Zhang, H. Shioyama,
N. Kuriyama and Q. Xu, Int. J. Hydrogen Energy, 2009, 34,
2303–2311.
2
M. Ramzan, F. Silvearv, A. Blomqvist, R. H. Scheicher,
S. Lebeque and R. Ahuja, Phys. Rev. B: Condens. Matter
Mater. Phys., 2009, 79, 132102–132104.
Q. Xu and M. Chandra, J. Power Sources, 2006, 163, 364–370.
G. Wolf, J. Baumann, J. Baitalow and F. P. Hoffmann,
Thermochim. Acta, 2000, 343, 19–25.
Conclusions
In summary, rhodium(0) nanoparticles supported on titania
were reproducibly prepared from the reduction of Rh @TiO
during the catalytic hydrolysis of ammonia borane. Rhodiu-
m(III) ions were impregnated on titanium dioxide from the
aqueous solution of rhodium(III) chloride and then reduced by
3
4
3
+
2
5
6
7
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5 S. Akbayrak and S. Ozkar, ACS Appl. Mater. Interfaces, 2012, 4,
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