G Model
APCATA-15330; No. of Pages11
ARTICLE IN PRESS
J. Lee et al. / Applied Catalysis A: General xxx (2015) xxx–xxx
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advances in nanotechnology allow us to prepare nano-faceted
Al O with systematic variations in their morphologies [2,19–25].
Thus, the prepared crystallites have different relative ratios of
specific facets allowing systematic studies aimed at finding corre-
using a 2.0% ethanol/He gas mixture (1.0 ml/s), followed by a
He purge for 30 min in order to remove weakly-bound ethanol
molecules. After stabilization of the flame ionization detector (FID)
signal of an Agilent 7820A gas chromatograph (GC), a TPD exper-
iment was carried out in flowing He (1.0 ml/s) with a heating rate
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lations between phase transformations of ␥-Al O3 and crystallite
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morphologies that otherwise would be very difficult with commer-
of 10 C/min, and the reactor outlet flowing directly to the FID (i.e.,
cial ␥-Al O . These information will be very useful for potential
no GC column separation).
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development of thermally stable ␥-Al O without any surface mod-
All solid state 27Al-NMR experiments were performed at room
temperature on a Varian VNMRS 600 MHz FT-NMR spectrometer,
operating at a magnetic field of 14.4 T. The corresponding 27Al Lar-
mor frequency was 156.299 MHz. All the spectra were acquired at
a sample spinning rate of 25 kHz, using a 1.6-mm pencil-type MAS
probe. Each spectrum was acquired using a total of 2000 scans with
a recycle delay time of 1 s. All spectra were externally referenced
(i.e., the 0 ppm position) to a 1 M Al(NO ) aqueous solution. We
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ifier such as La, Ba, etc., that can affect the catalytic properties
[
11,13–16,26–29].
In this study, we investigated the phase transformation of
platelet- and rod-shaped ␥-Al O [21] by XRD, BET, ethanol TPD,
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solid state Al-NMR and HR-TEM after sequential calcinations in
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air up to 1100 C. After 1100 C treatment, commercial ␥-Al O
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transformed to ␣-Al O while platelet transformed to the -phase
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3 3
and rod transformed to the ␦-phase. Furthermore, platelet- and
normalized the 27Al MAS spectra with the same total NMR peak
area for the ease of comparison.
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rod-shaped alumina showed much higher surface areas (60 m /g)
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than commercial ␥-Al O3 (25 m /g) after 1100 C annealing. In
Transmission electron microscopy (TEM) images were taken
with a JEM-2100 unit operated at 200 kV. A TEM samples were
prepared by dropping alumina particles onto a glass slide, and the
particles were carefully ground between two glass slides. By slid-
ing the carbon coated Cu grid onto the glass slide, ground particles
were transferred onto the Cu grid.
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addition to higher thermal stabilities of platelet- and rod-shaped
aluminas in comparison to commercial alumina, they showed dif-
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ferent catalytic properties after 1100 C calcination. These results
consistently suggest the morphology-dependent phase transfor-
mations of ␥-Al O and improved thermal stabilities of platelet-
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and rod-shaped Al O in comparison to a commercial ␥-Al O .
Steady-state ethanol dehydration reaction was performed in a
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quartz flow reactor using 0.01 g samples supported by quartz wool.
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Samples were treated in flowing 20% O /He at 500 C for 1 h. The
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. Experimental
Platelet- and rod-shaped ␥-Al O3 samples used for this study
carrier gas (He) was passed through a bubbler containing ethanol
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(99.5%, Sigma–Aldrich) kept at 23 C and ambient pressure. The
ethanol concentration (2%) was controlled by relative He flow rate
(total flow rate of He was 1.0 ml/s). The outlet gases were ana-
lyzed by a GC (Agilent 7820A) using a HP-FFAP column and FID.
Catalytic activities were evaluated under conditions where ethanol
conversion was kept below 10%.
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were synthesized based on a previous report [21]. Typical syn-
thetic procedure for rod-shaped ␥-AlOOH was as follows. First,
Al(NO ) ·9H O (24.9 g) was dissolved in distilled water (400 ml)
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to form a transparent solution. Then, hydrazine monohydrate
(
N H ·H O, 10.8 g) was dropped into the solution, giving rise to
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a milky precipitate at pH = ∼5. The resultant reaction mixture was
transferred into a 500 ml Teflon-lined autoclave, which was then
3. Results and discussion
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sealed and kept in the electric oven at 200 C. After 12 h, the
resultant precipitates were collected by centrifugation, repeatedly
washed by DI water and isopropyl alcohol, and dried in air at 100 C
We synthesized platelet- and rod-shaped aluminas using the
synthesis procedure shown in Fig. 1(a). After hydrothermal treat-
ment at 200 C, platelet- and rod-shaped boehmite (AlOOH) phases
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for 12 h. Platelet ␥-AlOOH were also synthesized using the same
procedure except that the initial pH value of the reaction mixture
was controlled to 9–10. The as-prepared-AlOOH samples were cal-
were confirmed by XRD (␥-AlOOH, JCPDS 21-1307), which are
shown in Fig. 1(b). Platelet- and rod-shaped boehmites showed
identical XRD patterns, but morphological differences were con-
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cined in a muffle furnace at 600 C for 3 h, resulting in the platelet-
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and rod-shaped ␥-Al O .
firmed by TEM analysis discussed later. After calcination at 600 C,
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Three ␥-Al O samples, commercial (Puralox SBA-200 from
both platelet- and rod-shaped boehmites transformed to ␥-Al O3
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Sasol), platelet- and rod-shaped alumina were used in this work.
Both platelets and rods were synthesized by the synthesis proto-
that was also confirmed by XRD patterns shown in Fig. 1(c). The
obtained XRD patterns are consistent with previously reported XRD
patterns of ␥-Al O3 and also commercial ␥-Al O3 (Puralox SBA-
cols described above. Each ␥-Al O3 samples (0.5 g) were calcined
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in a muffle furnace in air atmosphere simultaneously at specific
temperatures for 3 h. The annealing temperature was increased
stepwise from 600 C to 1100 C in 100 C intervals.
200) [31].
We need to point out here that the synthesis protocols used in
this study for the preparation of the rod and platelet shaped alu-
mina samples and the subsequently extensive rinsing with water
and isopropyl alcohol ensured that no alkali impurities were left in
the alumina samples. It has been well documented that impurities
such as alkali metals (e.g., Na ) can substantially modify the thermal
stabilities of ␥-Al O3 and also influence its phase transformations
[11,13,14,26–29].
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After the synthesis, X-ray diffraction (XRD) analysis was car-
ried out to confirm the phase of the alumina. XRD patterns
were obtained on a Bruker D8 Advance using Cu K␣ radiation
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+
(
ꢀ = 1.5406 A˚ ) in step mode between 2ꢁ values of 5 and 75 , with
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a step size of 0.05 /s. The specific surface areas of the alumina
samples were determined by the BET method using an automated
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adsorption instrument (ASAP2420). Prior to N adsorption, the
samples were treated in dry N2 flow at 150 C for 4 h.
To characterize surface properties of the thermally treated alu-
mina samples, ethanol TPD experiments were carried out using the
same experimental procedures as described in our previous report
Morphologies of the synthesized alumina samples were con-
firmed by TEM analysis (Fig. 2). Panels (a) and (b) show
as-synthesized boehmites of typical platelet with a rhombus
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shape and rod-shaped morphologies. After calcination at 600 C,
boehmites were converted to ␥-alumina which shows the same
platelet- and rod-shaped morphologies, as shown in Fig. 2(d)
a morphology of agglomerated particles with irregular shapes.
In Fig. 2(d), ␥-alumina particles formed from platelet boehmites
[
30]. Prior to ethanol TPD experiments, 0.05 g of pre-treated alu-
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mina was annealed again at 500 C for 1 h under He flow (1.0 ml/s).
After calcination, the sample was cooled down to room temper-
ature (23 C), and ethanol adsorption was carried out for 30 min
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