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5092
J. Phys. Chem. A 2010, 114, 5092–5098
Photocatalytic Degradation of Chlorinated Ethanes in the Gas Phase on the Porous TiO2
Pellets: Effect of Surface Acidity
Suzuko Yamazaki,* Keiko Ichikawa, Atsue Saeki, Toshifumi Tanimura, and Kenta Adachi
DiVision of EnVironmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi
UniVersity, Yamaguchi 753-8512, Japan
ReceiVed: December 15, 2009; ReVised Manuscript ReceiVed: March 12, 2010
The photocatalytic degradation of chlorinated ethanes was studied in a tubular photoreactor packed with
TiO2 pellets prepared by sol-gel method. The steady-state condition was not obtained, but the deterioration
in the photocatalytic activity was observed during the irradiation. Effects of mole fractions of water vapor,
O2, and C2H5Cl or C2H4Cl2 and reaction temperature on the photodegradation of C2H5Cl or C2H4Cl2 were
examined, and these data were compared with those obtained by the photodegradation of chlorinated ethylenes.
On the basis of the products detected with and without oxygen in the reactant’s gas stream, we proposed the
degradation mechanism. Measurement of diffuse reflectance infrared Fourier transform spectroscopy of pyridine
adsorbed on the catalysts showed that decrease in the conversion for the photodegradation of C2H5Cl was
attributable to the formation of Brønsted acid sites. Comparison of the data obtained with the TiO2 and the
sulfated TiO2 (SO42-/TiO2) pellets indicated that the photodegradation of C2H5Cl was suppressed by the
presence of the Brønsted sites, but that of trichloroethylene was not affected. Such a difference is attributable
to the adsorption process of these reactants on the acid sites on the catalyst surface.
Introduction
photocatalytic degradation of chloroform.16 To improve the
usefulness of the TiO2 photocatalyst, it is essential to understand
the surface chemistry. In this study, we performed the photo-
catalytic degradation of chloroethanes to compare the data with
those of chloroethylenes. We describe that the photocatalytic
activity of the TiO2 decreases drastically during the photodeg-
radation of chloroethanes, and surface acidity on the catalyst
affects the photocatalytic performance.
Volatile chlorinated organic compounds (VCOCs) such as
trichloroethylene (TCE) and trichlroethane have presented
serious environmental concerns for several decades because they
are toxic and carcinogenic to animals. Several methods are
available to remove VCOCs from waste gas, such as adsorption
on activated carbon, wet scrubbing, and catalytic oxidation. The
former two methods only transfer VCOCs from gaseous phase
to solid or liquid phase, whereas catalysis can destroy them.
Photodegradation of various pollutants by photocatalysis,
using wide band gap semiconductors, has been widely studied.
Among them, the TiO2 photocatalysts have been of great interest
because TiO2 is stable, harmless, and inexpensive and potentially
can be activated by solar energy.1 In particular, studies on the
gas-phase photocatalytic degradation of TCE have become an
active field because many soils and groundwater supplies have
been contaminated with TCE as a result of leaking underground
storage tanks and improper disposable practices. The photo-
catalytic degradation of TCE to CO2 and HCl on the TiO2 has
been studied by many researchers, and dichloroacetyl chloride,
phosgene, and chloroform have been identified as byproducts.2-13
Most of these studies were conducted in a closed vessel.
However, for developing as a practical remediation method for
a contaminated gas, a flow system in a noncirculating mode,
that is, single pass mode, is more appropriate. We performed
kinetic studies on the photocatalytic degradation of chlorinated
ethylenes such as ethylene chloride, TCE, and tetrachloroeth-
ylene (PCE) on the porous TiO2 pellets prepared by sol-gel
method in the single pass mode and demonstrated that undesir-
able byproducts were produced via Cl radical initiated
mechanism.14,15 For these chloroethylenes, the catalytic activity
of the TiO2 pellets was maintained, whereas it decreased
remarkably with an increase in the irradiation time for the
Experimental Methods
Ethane, chloroethane, or 1,2-dichloroethane (500 ppmv,
balance nitrogen, Sumitomo Seika Chemicals Co.), nitrogen
(99.999%), oxygen (99.999%), and synthesized air (oxygen 20.0
to 21.5%) were used as received from compressed gas cylinders.
Humidified air was prepared by bubbling gas through a glass
saturator containing deionized water. Water content was fixed
as a result of controlling temperature and the flow rate of the
gas stream passing through the saturator. Porous TiO2 pellets
were prepared by sol-gel techniques and fired at 200 °C.8 The
specific surface area and porosity of these materials were 154
m2 g-1 and 50%, respectively, as obtained by the BET analysis.
The photodegradation experiments were carried out in a
packed bed tubular photoreactor (Pyrex, 11.5 cm long, 0.24 cm
i.d., and 0.32 cm o.d.) in a noncirculating mode. Four 4 W
fluorescent black light bulbs (Toshiba, FL 4BLB) surrounded
the tubular reactor. The photon flow rate (wavelength <400 nm)
entering the reactor was determined to be 1.7 × 10-7 einstein
s-1 using a uranyl oxalate actinometer.8 All experiments except
for examining an effect of temperature were conducted at room
temperature, and air was passed around the reactor to avoid the
temperature of the pellets from rising by illumination.
The concentrations of ethane, chloroethane, and 1,2-dichlo-
roethane as reactants and CO2, ethylene, and vinyl chloride as
products in the gas stream were analyzed by gas chromatography
(Hewlett-Packard 5890 series II equipped with a Porapak R
* Corresponding author. Tel/Fax: +81-83-933-5763. E-mail: yamazaki@
yamaguchi-u.ac.jp.
10.1021/jp911842t 2010 American Chemical Society
Published on Web 03/25/2010