R.T. Yunarti et al. / Catalysis Communications 50 (2014) 54–58
55
mixed oxides may exhibit the good catalytic OCM activity, and we elu-
3. Results and discussion
cidate the effects of transition metal dopants on the OCM activity in
this study.
3.1. Catalyst preparation
2
. Experimental section
2
TiO nanowire catalysts were prepared using a molten-salt synthesis
method at 825 °C, as described in the literature [21]. Eutectic salt mix-
tures with appropriate compositions were used to prepare the catalysts.
Transition metal (TM) dopants, including V, Cr, Mn, Co, Mo, and Rh,
2
.1. Materials
All chemicals were used without further purification unless other-
were added during the synthesis of the TiO
2
nanowires to tune the
) nanowires.
wise indicated. TiO
Germany). Sodium chloride (NaCl, 99.5%), sodium phosphate dibasic
Na HPO , 99.0%), manganese (II) nitrate tetrahydrate (Mn(NO .4H O,
7%), rhodium (III) nitrate hydrate (Rh(NO · xH O), sodium tungstate
dihydrate (Na WO .2H O, 99%), and vanadium (V) oxide (V , 99.99%)
were purchased from Aldrich (Milwaukee, Wisconsin, USA). Cobalt
2
powder (P25) was purchased from Evonik (Essen,
catalytic activities of the TM-doped TiO (TM-TiO
2
2
The morphologies of prepared nanostructures were observed by
means of scanning electron microscopy (SEM), which indicated that
TM doping did not significantly change the morphology of the TiO
nanowires (Figs. 1 and S1). Most nanowires were 1–40 μm long and
~100 nm wide regardless of the TM doping, although the Rh-TiO nano-
(
9
2
4
3
)
2
2
3
)
3
2
2
2
4
2
2 5
O
2
(
II) nitrate hexahydrate (Co(NO
heptamolybdate tetrahydrate (Mn(NO
nese nitrate hexahydrate (Mn(NO
from Junsei Chemical (Tokyo, Japan). Chromium (III) nitrate
enneahydrate (Cr(NO .9H O, 97.0%) was purchased from Kanto
3
)
2
.6H
2
O, 97.0%), hexaammonium
.4H O, 99.0%), and manga-
O, 98%) were purchased
wires were smaller than the others. The nanowires had the negligible
3
3
)
2
2
pore volumes (0.013 0.038 cm /g) and appreciable BET surface areas
2
3
)
3
.6H
2
(7.5 12.1 m /g) (Fig. S2 and Table S1), indicating that catalysis occurred
only on their external surfaces. In addition to the morphologies of the
catalysts, the crystal structures of the nanowires were observed with
3
)
3
2
Chemical (Tokyo, Japan). DI water (18.2 MΩ cm) was prepared
using an aqua MAX-Ultra 370 Series water purification system
X-ray diffraction (XRD) (Figs. 2 and S3). P25 TiO
mostly an anatase phase of TiO , which transformed into a rutile
phase during the synthesis of the nanowires at 825 °C [22]. Although
the TMs were incorporated, the TM-TiO nanowires also exhibited a
rutile phase of TiO , with no appreciable change in the XRD patterns ob-
served in the XRD, which indicated that TM doping did not significantly
distort the crystal structures of the TiO nanowires and that the change
crystal structures did not adjust the catalytic activity in this
2
particles exhibited
2
(
Young Lin Instrument, Anyang, Korea). Methane (CH
ygen (O , 99.5%), and helium (He, 99.999%) were purchased from
Shinyang Sanso (Seoul, Korea).
4
, 99.97%), ox-
2
2
2
2
.2. Catalyst preparation
2
in the TiO
study.
2
Undoped TiO
nanowires were prepared following a method proposed in the literature
21]. In order to prepare undoped TiO nanowires, a mixture of P25,
NaCl, and Na HPO at a weight ratio of 1:4:1 was ground using a mortar
and pestle to form a fine powder, which was calcined in a furnace at
25 °C for 8 h. Cooling to room temperature, the calcined mixture was
washed with an excessive amount of boiling DI water to remove all sol-
uble salts. TM-doped TiO nanowires were prepared by adding 2% (by
atomic percentage) of transition metal (V, Cr, Mn, Co, Mo, and Rh)
into the initial mixture of P25, NaCl, and Na HPO . Among the transition
2 2
nanowires and transition metal (TM)-doped TiO
[
2
3.2. Catalytic activity measurement
2
4
8
2
The undoped and TM-doped TiO nanowire catalysts were used in
OCM reactions and their activities were assessed by measuring the C
(ethane and ethylene) yield (Fig. 3, Tables S2, and S3). The undoped
TiO nanowires exhibited the poor catalytic activity as predicted from
our previous works [9,23]. Among the TM-doped nanowire catalysts,
Mn-TiO and Rh-TiO nanowires exhibited high methane conversions
of 13.5 20.1% (Mn-TiO ) and 26.0 30.4% (Rh-TiO ), respectively. V-TiO
and Co-TiO also exhibited higher conversions compared to those of
undoped TiO
2
2
2
2
4
metals, the concentration of Mn (2–8% by atomic percentage) was ad-
justed to optimize the catalyst.
2
2
2
2
2
2
2
.3. Catalyst characterizations
2
nanowires, but the Cr-TiO
2
and Mo-TiO
2
nanowires
The XRD results were obtained using a Shimadzu (Tokyo, Japan)
XRD-6000 equipped with a CuKα (λ = 0.15406 nm) source. Scanning
1
electron microscopy (SEM) of the catalysts was performed using a
NovaSEM 200 to observe the morphology of the catalysts. UV–vis spec-
trometry was performed using a UV–vis-NIR infrared spectrophotome-
ter (Cary 5000, Varian, USA). The BET (Brunauer–Emmett–Teller)
surface areas and the pore structures of the catalysts were observed
using an ASAP 2020 (Micromeritics, Norcross, GA, USA) instrument
after degassing the catalyst powder at 120 °C for 12 h under vacuum.
X-ray photoelectron spectroscopy (XPS) was performed using an
Excalab 250 (Thermofisher Co., UK, KBSI-PA311).
2
.4. Catalytic activity measurements
The catalyst placed in a cylindrical quartz reactor was treated under
a N
2
flow (30 mL/min) at 700 °C for 1 h prior to the reaction. A flow
)/(O ) =
passed the catalyst bed at reaction temperatures of
50, 775, 800, 825, and 850 °C. The pressure drop was monitored
(
30 mL/min) of mixed reactants, methane and oxygen ((CH
4
2
5
), diluted with N
2
7
using a pressure gauge set at the entrance to the reactor. The reaction
products passed the condenser at −2 °C to remove the water vapor pro-
duced during the reaction prior to being observed with a GC–FID (flame
ionization detector) and a GC–TCD (thermal conductivity detector).
Fig. 1. SEM images of (a) TiO
2
, (b) V-TiO
2
, (c) Mn-TiO
2
, and (d) Rh-TiO
2
nanowires.