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ARTICLE IN PRESS
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E.J. Park et al. / Catalysis Today xxx (2015) xxx–xxx
of 250 ◦C and 350 ◦C were chosen: 350 ◦C is a temperature,
which ensures complete total oxidation of toluene using catalysts,
whereas at 250 ◦C, non-complete total oxidation and significant
change in the toluene oxidation activity as a function of reaction
time can be observed. These are also in the range of typical work-
ing temperatures of NiO-based catalyst for toluene oxidation [25].
Moreover, strategies for developing humidity-insensitive catalysts
by modulating substrate surface structure will be discussed.
2. Experimental
2.1. Sample preparation
NiO/SiO2 and NiO/TiO2/SiO2 catalysts were prepared by atomic
layer deposition (ALD) method, and Table 1 shows the detailed
parameter of ALD process used in this study. Prior to the ALD pro-
cess, the mesoporous SiO2 (average pore size = 13.8 nm, Aldrich)
was outgassed at 500 ◦C for 3 h under ambient pressure condi-
tions. For the deposition of NiO, 0.6 g of outgassed mesoporous SiO2
was placed in an ALD reactor and exposed to bis(cyclopentadienyl)
nickel (Ni(Cp)2, Aldrich) vapor with a working pressure of 200
mtorr and O2 with 1.3 torr for 30 and 40 s, respectively, in an alter-
nate manner. During the ALD process, the sample temperature was
maintained at 260 ◦C, whereas those of Ni(Cp)2 and O2 bottles at
60 ◦C and room temperature, respectively.
NiO/TiO2/SiO2 catalyst was prepared by depositing TiO2 first and
subsequently NiO on the outgassed mesoporous SiO2 using ALD.
For TiO2 deposition, titaniumtetraisopropoxide ([Ti(OCH(CH3)2)4],
TTIP, Aldrich) and H2O were used as precursors of Ti and O, respec-
tively. The sample temperature was kept at 150 ◦C, whereas TTIP
and H2O bottles at 50 ◦C and room temperature, respectively, dur-
ing ALD of TiO2. TTIP precursor with a working pressure of 400
mtorr was exposed to the sample for 40 s and subsequently H2O
with 400 mtorr for 50 s. After the TiO2 deposition, the sample was
transferred to another ALD reactor for NiO deposition using the
same method as that for the preparation of NiO/SiO2. In all the ALD
processes, the base pressure was maintained below 10 mtorr and
high purity N2 (99.999%) gas was used as purging gas between the
precursor exposing steps. 40 and 50 cycles of ALD were used for the
deposition of NiO and TiO2, respectively.
Figure 1. Experimental set-up for the toluene oxidation experiments under dry and
humid conditions at various temperatures is schematically illustrated.
analyses, respectively. All of the analyses were carried out on the
samples outgassed at 500 ◦C for 90 min under a constant flow of air.
Catalytic activities of NiO/SiO2 and NiO/TiO2/SiO2 for toluene
combustion were evaluated under dry and humid conditions.
Figure 1 shows a schematic diagram of experimental set-up for
toluene combustion with a humidity control system. The set-up
consisted of the gas preparation stage, the reactor with furnace
and gas chromatograph (GC, Agilent-6890 N) equipped with meth-
anizer and flame ionization detector (FID). The gas preparation
stage was composed of three mass flow controllers (MFCs) and
two bottles filled with toluene (Aldrich, purity 99.8%) and distilled
water, respectively. Temperatures of toluene and distilled water
bottles were maintained at 30 ◦C, and MFC 1, 2 and 3 provided con-
stant flows of dry air, humid air and dry air mixed with toluene
to the reactor, respectively. 0.5 g of a catalyst was packed in the
quartz reactor using quartz wool and outgassed at 500 ◦C for 90 min
under a flow of 20 sccm of dry air to remove impurities of the cat-
alyst. Then, the temperature of the reactor was cooled down to
either 250 ◦C or 350 ◦C for toluene combustion experiments. After
the temperature of the reactor was stabilized, either dry or humid
airs containing toluene vapor with a concentration of ∼ 80 ppm was
introduced into the reactor with a total flow of 20 sccm and the flow
was maintained for 10 h. The relative humidity of the reactor was
less than 1% for the dry condition and 70% for the humid ones.
After the toluene combustion experiment, temperature pro-
grammed oxidation (TPO) was subsequently conducted in order
to identify the amount of carbon species remaining on the catalyst
surface as a result of molecular adsorption of toluene and its par-
tial oxidation. Each TPO experiment was conducted from 150 ◦C to
500 ◦C with a ramping rate of 1 ◦C/min under a constant dry air flow
of 5 sccm.
2.2. Sample characterization
The elemental compositions of NiO/SiO2 and NiO/TiO2/SiO2
catalyst were analyzed by inductively coupled plasma-optical
emission spectroscopy (ICP-OES, 720-ES, Agilent) and the surfaces
of the catalysts were analyzed using x-ray photoelectron spec-
troscopy (XPS) equipped with concentric hemispherical analyzer
(CHA, PHOIBOC-Has 2500, SPECS) and monochromatic X-ray source
(Mg K␣, 1253.6 eV). The morphological images of samples were
obtained using scanning transmission electron microscopy (STEM,
JEM-3010, JEOL). The N2 adsorption-isotherm was used to evalu-
ate the surface area and pore size distribution of samples based on
the BET (Brunauer-Emmett-Teller) and BJH (Barett-Joyer-Halenda)
Temperature programmed desorption (TPD) experiments were
also carried out in order to evaluate the toluene uptake under dry
and humid condition. 0.1 g of each sample was outgassed at 500 ◦C
for 90 min and the samples were cooled to 30 ◦C. The outgassed
Table 1
Experimental parameters used for ALD of NiO and TiO2 are summarized.
NiO ALD process
TiO2 ALD process
Precursors
Ni(Cp)2 at 200 mtorr
230 s
O2 at 1.3 torr
40 s
TTIP at 400 mtorr
42 s
H2O at 400 mtorr
50 s
Precursors exposure time
N2 purging time
Pumping time
Substrate temperature
Reactor base pressure
30 s
60 s
20/30 s
40/60 s
260 ◦
C
150 ◦
C
10 mtorr
10 mtorr
Please cite this article in press as: E.J. Park, et al., Toluene oxidation catalyzed by NiO/SiO2 and NiO/TiO2/SiO2: Towards development of