Enhanced Photocatalytic Activity of Ce
3+
-TiO
2
for 2-Mercaptobenzothiazole
Degradation in Aqueous Suspension for Odour Control
F. B. Li
1, 2
, X. Z. Li
1*
, M. F. Hou
2
, K. W. Cheah
3
and W. C. H. Choy
3
1
Department of Civil and Structural Engineering, The Hong Kong Polytechnic University,
Hong Kong, China
2
Guangdong Key Laboratory of Agricultural Environment Pollution Integrated Control, Guangdong
Institute of Eco-Environment and Soil Science, Guangzhou, 510650, China
3
Department of Physics, Hong Kong Baptist University, Hong Kong, China
Abstract: A series of cerium ion-doped titanium dioxide (Ce
3+
-TiO
2
) catalysts with special 4f
electron configuration was prepared by a sol-gel process and characterized by Brunauer-Emmett-
Teller method, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), UV-visible diffuse
reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The
photocatalytic activity of Ce
3+
-TiO
2
catalysts was evaluated in the 2-mercaptobenzothiazole (MBT)
degradation in aqueous suspension under UV or visible light illumination. The experimental results
demonstrated that the overall photocatalytic activity of Ce
3+
-TiO
2
catalysts in MBT degradation was
signigicantly enhanced due to higher adsorption capacity and better separation of electron-hole pairs.
The experimental results verified that both the adsorption equilibrium constant (K
a
) and the saturated
adsorption amount (
max
) increased with the increase of cerium ion content. The results of XPS
analysis showed that the Ti
3+
, Ce
3+
, and Ce
4+
ions reside in the Ce
3+
-TiO
2
catalysts. The results of
DRS analysis indicated that the Ce
3+
-TiO
2
catalysts had significant optical absorption in the visible
region between 400-500 nm because electrons could be excited from the valence band of TiO
2
or
ground state of cerium oxides to Ce 4f level. In the meantime, the dependence of the electron-hole
pair separation on cerium ion content was investigated by the PL analysis. It was found that the
separation efficiency of electron-hole pairs increased with the increase of cerium ion content at first
and then decreased when the cerium ion content exceeded its optimal value. It is proposed that the
formation of two sub-energy levels (defect level and Ce 4f level) in Ce
3+
-TiO
2
might be a critical
reason to eliminate the recombination of electron-hole pairs and to enhance the photocatalytic
activity.
Keywords: Adsorption; Cerium ion; Titanium dioxide; 2-mercaptobenzothiazole; Visible light
*
To whom correspondence should be addressed. Fax: (852) 2334 6389; Email: cexzli@polyu.edu.hk
This is the Pre-Published Version.
1. Introduction
TiO
2
-based photocatalytic oxidation techniques have received much attention thanks to their
application potential for complete mineralization of many toxic and non-biodegradable organics [1-
3]. Some studies indicated that the photocatalytic activity of TiO
2
catalysts depends strongly on two
factors: adsorption behavior and the separation efficiency of electron-hole pairs [1-3]. The adsorption
capacity can be generally improved by adjusting the surface-zero-charge point or by increasing the
specific surface area and pore volume of catalysts [4-6].
On the other hand, it has been reported that
doping with a group of transitional metal ions [5-11] or depositing some noble metals such as Au [4,
12-14] and Pt [15], or coupling metallic oxides [16-19] with d electronic configuration into TiO
2
lattice could eliminate the recombination of electron-hole pairs significantly and also result in the
extension of their wavelength response toward the visible region [20].
Alternatively, the photocatalytic activity of TiO
2
could be significantly enhanced by doping with
lanthanide ions/oxides with 4f electron configurations because lanthanide ions could form complexes
with various Lewis bases including organic acids, amines, aldehydes, alcohols, and thiols in the
interaction of the functional groups with their f-orbital [21-22]. Ranjit et al. [21-22] reported the
increase of saturated adsorption capacity and the adsorption equilibrium constant simultaneously for
salicylic acid, t-cinnamic acid, and p-chlorophenoxy-acetic acid owing to the Eu
3+
, Pr
3+
, or Yb
3+
doping. Xie et al. [23] reported Nd
3+
-TiO
2
sol catalysts had photocatalytic activity for phenol
degradation under visible light irradiation. However, the effect of lanthanide oxides on the separation
of electron-hole pairs under visible light irradiation and the photoresponse had seldom been
investigated in these publications. Among the lanthanide oxides, the catalytic properties of ceria have
received much attention due to two features of (1) the redox couple Ce
3+
/Ce
4+
with the ability of
ceria to shift between CeO
2
and Ce
2
O
3
under oxidizing and reducing conditions; and (2) the easy
formation of labile oxygen vacancies (OV) with the relatively high mobility of bulk oxygen species
[18]. Furthermore, the different electronic structures of the Ce
3+
with 4f
1
5d
0
and the Ce
4+
with 4f
0
5d
0
would lead to different optical properties [24-25] and dissimilar catalytic properties for CeO
2
and
CeO
x
-TiO
2
and also Ce
3+
-TiO
2
[26-27].
In this study, Ce
3+
-TiO
2
catalysts were prepared by a sol-gel method and 2-mercaptobenzothiazole
(MBT) was used as a model chemical to carry out the photocatalytic activity tests under UV or
visible light irradiation. MBT is an odorous chemical with a –SH group and isolated electron pairs,
which has no optical absorption band in the visible region. The aim of this study was at investigating
the effects of cerium ion doping on (1) investigating the contribution of adsorption ability and the
separation of electron-hole pairs to the enhancement of apparent photocatalytic activity under either
UV or visible light irradiation; (2) eventually disclosing the mechanisms of photocatalytic activity
enhancement due to cerium ion doping.
2. Experimental Section
2.1 Preparation of Ce
3+
-TiO
2
Catalysts
The cerium ion-doped titanium dioxide (Ce
3+
-TiO
2
) catalysts were prepared by a sol-gel process with
the following procedure: 17 mL of tetra-n-butyl titanium (Ti(O-Bu)
4
) was dissolved into 80 mL of
absolute ethanol; then the Ti(O-Bu)
4
solution was added drop-wise under vigorous stirring into 100
mL of a mixture solution containing 84 mL of ethanol (95%), 1 mL of 0.1 M Ce(NO
3
)
3
, and 15 mL
of acetic acid (>99.8%); the resulting transparent colloidal suspension was stirred for 2 h and aged
for 2 d untill the formation of gel; the gel was dried at 253 K under vacuum and then ground to form
semisolid powder; the powder was calcined at 773 K for 2 h to form the Ce
3+
-TiO
2
powder
eventually. This Ce
3+
-TiO
2
catalyst had a nominal atomic ratio (Ce/Ti) of 0.2%, so it is named 0.2%
Ce
3+
-TiO
2
in this study. Accordingly, other Ce
3+
TiO
2
samples containing different cerium ion
contents were named 0.7% Ce
3+
-TiO
2
, 1.2% Ce
3+
-TiO
2
, and 2.0% Ce
3+
-TiO
2
, respectively.
2.2 Characterization of Ce
3+
-TiO
2
Catalysts
To determine the crystal phase composition of the prepared Ce
3+
-TiO
2
samples, we carried out X-ray
diffraction (XRD) measurements at room temperature using a Rigaku D/MAX-IIIA diffractometer
with CuK
radiation ( = 0.15418 nm). The accelerating voltage of 30 kV and emission current of 30
mA were used. The specific surface area of the catalysts was measured by the dynamic Brunauer-
Emmett-Teller (BET) method, in which a N
2
gas was adsorbed at 77 K using a Carlo Erba
Sorptometer. To study the light absorption of the catalysts, the diffuse reflectance spectra (DRS) of
the catalyst samples in the wavelength range of 200-800 nm were obtained using a UV-visible
scanning spectrophotometer (Shimadzu UV-2101 PC), while BaSO
4
was used as a reference. To
study the recombination of electrons/holes, the photoluminescence (PL) emission spectra of the
catalyst samples were measured with the following procedure: at either room temperature or 77 K, a
325 nm He-Cd laser was used as an excitation light source; the light from the sample was focused
into a spectrometer (Spex500) and detected by a photo-multiplier tube (PMT); the signal from the
PMT was inputted to a photon counter (SR400) before recorded by a computer. To study the valance
band state and chemical state of the photocatalysts, we received the X-ray photoelectron
spectroscopy (XPS) results of the catalyst samples with the PHI Quantum ESCA microprobe system,
using the MgK
line of a 250-W Mg X-ray tube as a radiation source with the energy of 1253.6 eV,
16 mA × 12.5 kV and the working pressure of lower than 1 × 10
-8
N m
-2
. As an internal reference for
the absolute binding energy, the C 1s peak at 284.80 eV of hydrocarbon contaminantion was used.
The fitting of XPS curves was analyzed with a software package (Multipak 6.0A).
2.3 MBT Adsorption Experiment
MBT chemical was provided by BDH and was used as a model odorous substrate in this study
without further purification. To determine the adsorption behavior of Ce
3+
-TiO
2
catalysts, we
performed a set of adsorption isotherm tests in the dark. In each test, to 10 mL of MBT suspension
was added 0.1 g of Ce
3+
-TiO
2
powder and the mixture was then put into a shaker operating at 130
revolutions min
-1
for 24 h at 298 ± 1 K. The MBT concentration in the suspension before and after
the adsorption tests was analyzed and the adsorbed amount of MBT on the catalysts was calculated
based on a mass balance.
2.4 MBT Photocatalytic Degradation Experiment
A Pyrex cylindrical photoreactor was used to conduct photocatalytic oxidation experiments, in which
an 8-W UVA lamp (Luzchem Research, Inc.) with a special emission peak at 365 nm was positioned
at the centre of the cylindrical vessel and was used for photoreaction under UV irradiation, while a
110-W high-pressure sodium lamp with main emission in the range of 400-800 nm was used for
photoreaction under visible light irradiation. The UV light portion of sodium lamp was filtered by 0.5
M NaNO
2
solution. This cylindrical photoreactor was surrounded by a Pyrex circulating water jacket
to control the temperature during the reaction. The reaction suspension was prepared by adding 0.25
g of Ce
3+
-TiO
2
powder into 250 mL of aqueous MBT solution. Prior to photoreaction, the suspension
was magnetically stirred in the dark for 30 min to approach adsorption/desorption equilibrium. The
aqueous suspension containing MBT and Ce
3+
-TiO
2
was aerated with a constant air flow. At the
given time intervals, the analytical samples were taken from the suspension and immediately
centrifuged for 20 min, and then filtered through a 0.45-m Millipore filter to remove particles.
2.5 Analytical Method
The MBT concentration in aqueous solution was determined by LC, which consists of a gradient
pump (Spectra System P4000), an autosampler (Spectra System Tem AS3000) with a 20 μl injection
loop, a Thermo Ques Hypersil ODS column (C18, 5 μm, 250 × 4.6 mm ID) and a photodiode array
UV detector (Spectra System UV6000LP). While a mobile phase (70% methanol and 30% water
with 1% acetic acid) was operated at a flow rate of 0.5 ml min
-1
, a maximum absorption wavelength
of 323 nm was selected to detect MBT.
3. Results and Discussion
