Inorganic Chemistry
Article
emission scanning electron microscope. BFTEM, HRTEM, STEM,
and HAADF-STEM samples were prepared by dropwise addition of
the dilute catalyst suspension onto a copper-coated carbon TEM grid,
followed by evaporation of the solvent. Conventional TEM
measurements were carried out on a JEOL JEM-200 CX transmission
electron microscope operating at 120 kV. HRTEM, STEM, and
HAADF-STEM analyses were performed using a JEOL JEM-2010F
transmission electron microscope operating at 200 kV. An Oxford
energy-dispersive X-ray (EDX) system and Inca software were
exploited to acquire and process the STEM-EDX data. The XPS
measurements were employed via a Physical Electronics 5800 XP
spectrometer equipped with a hemispherical analyzer and a
monochromatic Al Kα X-ray source (1486.6 eV, 15 kV, and 350 W,
with a pass energy of 23.5 eV). Fourier transform infrared (FTIR)
spectroscopy was carried out at room temperature by using a
Shimadzu IR-Affinity instrument using KBr disks in the 500−4000
cm−1 region. The solution NMR studies were carried out by using an
Afterward, this final solution was cooled to room temperature, and 50
mL of ethanol was added to separate solid Ir NPs by centrifugation
(10000 rpm for 10 min) and redispersed in hexane. The same
separation procedure was repeated two times by using ethanol, and
the resultant Ir NPs were collected by dispersion in hexane.
Synthesis of Bimetallic Ni@Ir Core−Shell NPs. A total of 0.25
mmol of Ir(acac)3 was dissolved in 10 mL of OAm and 2.0 mL of
OAc before Ni NPs (35 mg) in hexane (1 mL) were added. The
mixture was heated to 383 K under a N2 flow to remove hexane
before it was heated to 453 K within 1 h, and 2.5 mmol of KBEt3H in
1.0 mL of OAm was added to the mixture. The reaction mixture was
then cooled to room temperature. Next, the reaction solution was
cooled to room temperature, and the product was precipitated by the
addition of 30 mL of 2-propanol and collected by centrifugation
(10000 rpm for 10 min). The final product was dispersed in hexane.
Preparation of OMS-2 Nanorod-Supported Ni@Ir Core−
Shell NPs. A total of 20 mg of Ni@Ir core−shell NPs was dissolved
in hexane in a 20 mL vial, and an OMS-2 support was carefully added
to it. This colloidal mixture was first taken into sonication for 1 h to
ensure uniform distribution and then stirred for 2 h to ensure
complete adherence of Ni@Ir NPs to the OMS-2 nanorod support.
After evaporation of hexane, Ni@Ir/OMS-2 was collected as a solid
product, dried in a vacuum oven (10−1 Torr at 318 K) for 6 h, and
then kept in a glovebox before use. In order to remove surface-bound
surfactant groups, the Ni@Ir/OMS-2 catalyst was transferred into an
acetic acid solution and refluxed at 343 K for 12 h. Next, the Ni@Ir/
OMS-2 catalyst was separated by centrifugation (1000 rpm, 10 min),
washed with ethanol three times, dried in a vacuum oven (10−1 Torr
at 318 K) for 6 h, and then transferred into the N2-filled glovebox.
Measurement of the Catalytic Activity in the Dehydrogen-
ation of HB. The catalytic activity of Ni@Ir/OMS-2 in the
dehydrogenation of HB was determined by volumetric measurement
of the rate of gas evolution. The volume of released gas during the
reaction was monitored using a gas buret through water displacement
as described elsewhere.18−39 Before the catalytic activity tests were
started, a jacketed one-necked reaction flask (50.0 mL) containing a
Teflon-coated stirring bar was placed on a magnetic stirrer (Heidolph
MR-3004) whose temperature was adjusted by circulating water
through its jacket from a constant-temperature bath (Lab Companion
RW-0525). In a typical catalytic activity test, the Ni@Ir/OMS-2
catalyst was weighed and transferred into the reaction flask, and then
4.5 mL of H2O was added to the reaction flask, followed by rigorous
stirring for 15 min to achieve thermal equilibrium. Next, 0.5 mL of an
aqueous HB solution was added to the reaction flask via its septum
using a 1.0 mL gastight syringe, and the catalytic reaction was started
(t = 0 min) by stirring the mixture at 900 rpm. Gaseous products were
first passed through the HCl (0.1 M) trap to ensure NH3 absorption.
The volume of the remaining H2 along with N2 was monitored using
the gas buret through water displacement. Then, the molar ratio (λ)
was calculated as λ = n(H2 + N2)/n(N2H4BH3) by the total volume of
H2 and N2. The hydrogen selectivity (α) for N2H4BH3 dehydrogen-
ation (N2H4BH3 + 3H2O → B(OH)3 + (3 + 2α)H2 + (2α + 1)/3N2
+ 4(1 − α)/3NH3) was obtained by using the following
equation:27−37
1
Avance DPX 400 MHz spectrometer (400.1 MHz for H NMR and
100.6 and 128.2 MHz for 11B NMR). Si(CH3)4 and BF3·(C2H5)2O
were used as internal references for 1H and 11B NMR chemical shifts,
respectively.
Synthesis of HB. HB was synthesized by following the previously
reported synthesis protocols,52 in which in a 250 mL Schlenk tube
was stirred 100 mL of anhydrous dioxone containing 0.25 mol (20.3
g) of hydrazine hemisulfate salt and 0.25 mol (9.5 g) of sodium
borohydride at room temperature under a N2 atmosphere for 48 h.
Next, the resulting slurry was centrifuged at 10000 rpm for 5 min to
obtain a clear solution. Then, the final filtrate was evaporated by a
vacuum dryer at 40 °C for 12 h to obtain raw HB, which was further
washed with n-pentane. The purified sample was obtained as a white
1
powder with a purity of 99.5% verified by NMR spectroscopy. H
NMR (400.1 MHz, CD2Cl2): δ 5.2 (t, 2), 3.3 (b, 2), 1.1 (t, 3). 11B
NMR (128.2 MHz, H2O): δ 20 (q, 1). FTIR (selected, cm−1): 3310
(s), 3200 (s), 2840 (m), 2650 (m), 2370 (m), 2210 (m), 1610 (s),
1590 (m), 1440 (w), 1330 (m), 1150 (s), 910 (m), 750 (w), in
agreement with the literature values.52
Synthesis of OMS-2 Nanorods. OMS-2 nanorods were prepared
by following a procedure described in the literature.40 A potassium
permanganate solution in deionized water (0.4 M, 225 mL) was
added to a solution containing a mixture of a manganese sulfate
hydrate solution (1.75 M, 68.0 mL) and concentrated nitric acid (7.0
mL) in a 500 mL flask fitted equipped with a reflux condenser. The
resulting dark-brown slurry was refluxed for 24 h, then filtered, and
washed with deionized water several times. The catalyst was dried in a
vacuum oven under 10−1 Torr conditions at 393 K for 12 h before
use.
Synthesis of Monometallic Ni NPs. The synthesis of
monometallic Ni NPs was done by using a synthesis protocol
reported by Sun et al.53 In a typical synthesis protocol, 1.0 mmol of
Ni(acac)2 (258 mg) was added to a mixture of 15.0 mL of OAm and
0.4 mL of OAc in a glovebox. This mixture was then taken out of the
glovebox, heated to 383 K in 25 min, and kept at this temperature
under a N2 atmosphere for 1 h. Then, the solution was cooled to 368
K, and a 3.0 mL OAm solution of BTB (3.0 mmol) was injected into
the solution under a N2 atmosphere and kept at this temperature for
90 min. Next, this final solution was cooled to room temperature, and
50 mL of ethanol was added to separate Ni NPs by centrifugation
(10000 rpm for 10 min) as a solid product and redispersed in hexane.
In order to achieve better purification, the separation procedure was
repeated one more time by using ethanol, and the resultant Ni NPs
were collected by dispersion in hexane.
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n(H2 + N2)
n(N2H4BH3)
3λ − 10
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α =
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Catalytic Stability Measurements. The catalytic stability of the
Ni@Ir/OMS-2 catalyst in HB dehydrogenation was evaluated via
catalytic reusability experiments. The reusability of Ni@Ir/OMS-2 in
the dehydrogenation of HB was determined by performing a series of
experiments started with a aqueous HB solution at room temperature.
Instantaneously, after the achievement of HB conversion in the first
catalytic run, the catalyst was recovered by centrifugation (1000 rpm,
10 min), washed with a methanol/water (1/3, v/v) mixture three
times, and dried in a vacuum oven (10−1 Torr at 318 K). The dried
sample catalyst was weighed and used in the catalytic decomposition
of fresh HB. The same procedure was repeated up to a fifth catalytic
reuse.
Synthesis of Monometallic Ir NPs. In a N2-filled glovebox, 0.25
mmol of Ir(acac)3 was dissolved in a mixture of 10 mL of OAm and
0.4 mL of OAc placed in a three-necked round-bottomed flask
equipped with a condenser and a stir bar. Then, the mixture was taken
out of the glovebox and placed on a stirring heating mantle. Under a
N2 flow, this mixture was heated to 493 K and kept at this
temperature for 2 h. Then, the solution was cooled to 368 K, and a 3.0
mL OAm solution of BTB (1.25 mmol) was injected into the solution
under a N2 atmosphere and kept at this temperature for 90 min.
C
Inorg. Chem. XXXX, XXX, XXX−XXX
Bulut, Ahmet
