L. Saputra et al.
Molecular Catalysis 453 (2018) 132–138
Fig. 1. Self-regenerative Pd-contained perovskite as an “intelligent catalyst” [1,3].
a Rigaku Thermo plus Evo TG 8120 under a N
2
flow (250 mL min 1)
−
decane (99%), HNO
3
(60%) and molecular sieve (MS3A) pellets 3A 1/
−1
1
6 (pre-activated at 593 K for 6 h) were purchased from WAKO
Chemicals. Crotyl alcohol ( > 95%), 2-adamantanol ( > 98%), trans-
-hexenol ( > 95%), cyclooctanol ( > 98%), 4-tert-butylcyclohexanol
98%), and 2-octanol (98%) were purchased from TCI Chemicals. Ti
OBu) monomer (99%) was purchased from Kishida Chameleon
using Pt pans in the range of room temperature to 773 K (10 K min ).
ICP-OES measurements were performed using a Thermo Scientific iCAP
7000 Series equipped with an Autosampler ASX-260 CETAC (Pd:
340.4589 nm, K: 766.4904 nm, Sr: 407.7718 nm, Ti: 334.9411 nm). The
samples were dissolved in HCl 0.1 M prior to analysis.
2
(
(
4
Reagent. All solvents (WAKO: n-hexane (96%), cyclohexane (98%), n-
heptane (97%), toluene (99.5%), DMF (99.5%), 2-propanol (99.7%),
2.4. Catalytic test
1,4-dioxane (99.5%), TCI: trifluorotoluene) were distilled prior to use.
1
-Cyclohexylethanol (97%) and HCl (35%, for trace analysis grade)
Pd-STO (12.5 mg), 1-phenylethanol (25 mg, 0.204 mmol), n-decane
were purchased from Sigma Aldrich. Standard solutions (1000 ppm) of
Pd, K, Sr, and Ti were purchased from WAKO Chemicals.
as an internal standard and n-hexane (2 mL) as the solvent were placed
in a Schlenk flask equipped with reflux condenser, mechanical stirrer
and balloon. The reaction was carried out at approximately the reflux
2
.2. Preparation of the Pd-STO perovskite
temperature of n-hexane (342 K) under an O
4 h. In some cases, molecular sieves 3A (MS3A) were placed into the
reaction mixture.
2
atmosphere (0.1 MPa) for
2
The amorphous titania spheres (ATSs) were prepared by the direct
hydrolysis of Ti(OBu)
.7 g (5 mmol) of Ti(OBu)
lution A). A quantity of aqueous NH
solved in BuOH/MeCN (1:1 v/v) (Solution B). The molar ratio of H
was varied at 5.0, 12.5 and 25.0. The ATSs are denoted as ATS(x) with
x = H O/NH . The two solutions were preheated at 353 K for 10 min, then
mixed all together and stirred for 30 min. A white suspension was im-
mediately formed and precipitated. The white precipitate was rinsed with
ethanol and water and then dried at 348 K overnight.
4
, as described in our previous reports [13]. Briefly,
was dissolved in BuOH/MeCN (1:1 v/v) (So-
and distilled water were also dis-
O/NH
1
4
3
. Results and discussion
3
2
3
3.1. Characterization of the Pd-STO catalysts
2
3
All of the ATS have monodispersed spherical shapes, as shown in the
TEM (Fig. 2, a-c) and SEM (Fig. S3, a-c) images. Note that no pre-
cipitates were obtained in the absence of NH and/or H O. The average
3
2
particle sizes of ATS(5.0) and ATS(25.0) were approximately 0.5 μm,
Typically, the ATSs (2 mmol), Sr(OH)
lution (0.028 mmol) were poured into
2
·8H
a
2
O (2 mmol), and Pd so-
Teflon-lined autoclave
whereas the particle size of ATS(12.5) was approximately 1.5 μm. All of
2
the ATS contained microporous structures, as shown in the N isotherm
(
150 mL). Then, 20 mL of KOH 0.1 M and 80 mL of distilled water were
also poured into the autoclave. Depending on the ATS sources, the
perovskites were denoted as Pd-STO(x), with x = H O/NH . The mix-
plot (Fig. S2). Although the particle size of ATS(12.5) was relatively
larger than the others, the specific surface area (SBET) was significantly
higher (Table S1) due to the presence of meso- and/or macroporous
2
3
ture was homogenized under ultrasonic irradiation for 10 min. The
autoclave was placed into an oven for a hydrothermal reaction at 373 K
(
Fig. 2b, Fig. S3 b). These ATSs were then used for the preparation of
the Pd-contained perovskite.
After the hydrothermal reaction at 373 K, the particle size and shape
of the Pd-STO corresponded to the ATS sources (Fig. 2d-f). In addition
for 24 h. The obtained brown powder was rinsed with HNO
water, then dried at 348 K overnight. The chemical composition of the
Pd-STO(12.5) based on ICP-OES analysis was Pd1.0
.64 wt%). For comparison, a wetness impregnated catalyst (imp-Pd/
STO) was also prepared with equal Pd loading.
3
0.05 M and
K0.5Sr78.5Ti85.6 (Pd:
to the ATS sources, Pd-STO(12.5) also has the highest
S
BET
0
2 −1
(
17.3 m g ). This value was quite higher than the value resulting
2
−1
from the molten-salt method (≤11 m g ) [10]. Therefore, the H
2
O/
NH ratio during ATS preparation is crucial for tuning the particle size
3
2.3. Catalyst characterization
and porosity of the resulting Pd-STO perovskites. All of the ATS sources
could be used for perovskite formation, as shown in Fig. 3 c-e. The
The Pd-STO catalysts were characterized by a powder XRD MiniFlex
Rigaku instrument (Cu Kα, λ=1.5444 nm) with a Ni Kβ filter. The XRD
3
perovskite phase was confirmed and indexed to SrTiO (JCPDS card
35–734). The Pd-STO perovskite could be obtained after a hydro-
thermal reaction at 373 K (Fig. S4). To the best of our knowledge, this is
the lowest hydrothermal temperature for Pd-contained perovskite fab-
rication with tunable particle size and porosity. There were no other
phases observed, confirming the high purity of the resulting per-
ovskites. Additionally, we also used commercial TiO anatase and rutile
2
(not shown), but the perovskite phase was not formed, which proves the
necessity of an amorphous titania source.
−1
instrument operated at 40 kV and 15 mA with a scan rate of 10° min
and a scan step of 0.1°. SEM images were obtained using an SEM JEOL
JSM 6330F. N adsorption-desorption isotherms were obtained at 77 K
2
using a BELSORPmax instrument (BEL Japan). The samples were de-
gassed at 423 K for 2 h prior to the measurement. TEM images were
obtained using a Hitachi High-Tech H-7650 with an emissive gun, op-
erated at 150 kV. Thermogravimetric (TG) analyses were performed on
133