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photocatalyst have been investigated [21–23]. Synergistic effects
of microwave, leading to considerable increase in the pollutant
decomposition efficiency, were observed in those studies [24,25].
In this study, a TiO2 photocatalytic reaction process with
microwave assistance to enhance process efficiency was used to
process a non-biodegradable wastewater pollutants with a high
decomposition rate. In addition auxiliary oxidants (H2O2 and O2
gas) were added to enhance the response speed. The effects of
pH of the reactant aqueous solution and circulating fluid velocity
on the decomposition reaction were also examined. Intermediate-
products that were produced in the final decomposition reaction
were analyzed along with the decomposition reaction mechanism.
monitor to control the microwave intensity; and a microwave cav-
ity (470 × 550 × 235 mm), where photocatalysis reaction occurs.
The decomposition reaction experiment using the aforementioned
equipment is as follows. NB was dissolved in 500 mL of DI water;
thus, a 2 mM NB reactant aqueous solution was prepared. The
NB reactant solution was contained in a stainless steel beaker
installed in a constant temperature water bath and a roller pump
was used to circulate the photocatalyst reactor in the microwave
cavity at 200–500 cc/min fluid. When microwaves were irradi-
ated in the reactant aqueous solution, the temperature increased
continuously. To exclude the thermal effects of microwaves, the
reactant aqueous solution was cooled to 298 K using a constant
temperature water bath. Before the decomposition reaction, the
TiO2 catalyst balls was activated by ML for 30 s. In addition, before
the decomposition reaction, it was circulated for 5 min to maintain
the concentration of the NB reactant aqueous solution. Through
a wave-guide, the NB reactant aqueous solution was decomposed
when it was circulated in the microwave cavity with the existing
microwaves with an actual power of 0.2–0.6 kW. A three-stub tuner
was used to minimize the reflected microwaves and the inten-
sity was maintained using a power monitor. To evenly transfer the
microwaves within the microwave cavity, a stirrer was installed
inside the cavity.
2. Experimental
2.1. Chemicals
NB (>99.0%) and a H2O2 solution (30 wt.% in H2O) were pur-
chased from Sigma Aldrich Co. To control the pH, a hydrochloric
acid (0.1N) and sodium hydroxide (0.1N) solution obtained from
Daejung Chemicals and Metals Co. was used. Double distilled water
(Daejung Chemical & Metal Co.) was used to produce the response
aqueous solution, and all reagents were used as received.
2.2. TiO2 photocatalyst ball
2.5. Analysis
vapor deposition (CVD) was performed to coat a TiO2 film on the
surface of the alumina balls (diameter 8 mm, Nikkto, HD-11). Tita-
nium tetraisopropoxide (Ti(OC3H7)4) was used as the precursor at a
reaction temperature of 773 K for 1 h deposition. Detailed CVD pro-
cess conditions are reported elsewhere [27,28]. The top of Fig. 1a)
shows a cross section scanning electron microscopy (SEM) image of
the TiO2 photocatalyst ball destroyed by a hammer. Fig. 1b) shows
a SEM image of a magnified deposit of the TiO2 film on the sur-
face of the alumina ball. The thickness of the TiO2 film examined
in the TiO2 photocatalyst ball was approximately 1.2 m; Fig. 1c)
presents the photo-reactor comprised of quartz loaded with TiO2
photocatalyst balls; d) shows the X-ray diffraction (XRD) pattern of
the TiO2 film prepared by CVD. The TiO2 film had an anatase crystal
structure, which is oriented by the 112 face [27]. 300 TiO2 photo-
catalyst balls were used in this study, the total volume of TiO2 film
being 36.19 mm3.
The morphology of the TiO2 photocatalyst ball was exam-
ined by field emission scanning electron microscopy (FESEM,
JEOL-JSM-7100F), the crystal structure was characterized by
XRD (Max Science, MPX3). The pH of the NB reactant aque-
ous solution, which was controlled using HCl and NaOH, was
measured using a pH-meter (HM-30R, TOADKK). To measure
the decomposition rate according to the reaction time, sam-
ples of the NB reactant solution inside the stainless steel beaker
were collected and the concentration of NB was measured using
UV–vis spectrometer (UV-1801, Shimadzu Co.) at the maximum
wavelength (max = 268 nm). A gas chromatography mass spec-
trometer/headspace (GS/MS, QP2000, SHIMADZU Co. Ltd) auto
sampler was used to examine the NB decomposition mechanism
according to the microwave/ML/TiO2 ball hybrid photocatalyst sys-
tem and to review the intermediate-products. At this time, the
column used a HP-5 ms (30 m × 0.25 m × 0.25 mm) and reaction
was continued from the initial temperature of 323 K for 30 s, and
increased to 553 K at 10 K/min speed and then held at 553 K for
5 min.
2.3. Microwave lamp
TiO2 is activated by ultraviolet-rays and can conduct photo-
catalysis reaction. On the other hand, the UV-lamps ordinarily
used cannot be used in microwave field because of the adhered
metal electrodes. Therefore, in this study, a microwave investiga-
tion was conducted and a UV emitting electrodeless microwave
lamp (ML, 36 mm ID, 55 mm OD, and 170 mm length) was used.
The ML was prepared from double-tube type quartz and a min-
imal amount of mercury was charged within the vacuumed ML
tube. Fig. 2a) presents a photograph of the cavity with non-
irradiated microwave; Fig. 2b) shows a photograph of ML inside the
microwave cavity that is emitting UV and microwave radiation.
3. Results and discussion
3.1. Photocatalytic degradation of NB
Fig. 3 presents the change in the UV–vis spectrum accord-
ing to the reaction time of the NB reactant solution, which was
decomposed using the microwave/ML/TiO2 hybrid system. This
decomposition reaction was conducted under the condition that
the microwave intensity is a circulating reaction aqueous solution
at 0.5 kW with a 400 cc/min rate. The UV–vis spectrum shows that
the characteristic band can be observed in the NB reactant solu-
tion in wavelength at 268 nm [29]. Under increasing decomposition
reaction time, it clearly demonstrates decreased NB concentration
in the wavelength ranges at around 268 nm and approximately
99% of them was decomposed at 100 min. The absorbance of the
NB reactant solution (0 min) that did underwent the photocataly-
sis decomposition reaction at a wavelength 400 nm was 0. On the
other hand, when microwave/ML/TiO2 hybrid system was used to
conduct photo catalysis decompose reaction, absorbance increased
2.4. Experimental set-up
The NB decomposition reaction was conducted using
a
microwave/ML/TiO2 ball hybrid photocatalyst system. The hybrid
experimental system was composed the following elements: gen-
erator producing 2.45 GHz microwave (maximal power 1 kW); a
three-stub tuner for impedance matching, which was used to
maximize the transfer of electromagnetic waves energy; a power
Please cite this article in press as: S. Jeong, et al., Rapid photocatalytic degradation of nitrobenzene under the simultaneous illumination
Jeong, Sangmin
