Journal
of Chemical
andVladimir
Metallurgy,
52, 1,Alexander
2017, 13 Eliyas,
- 19 Ljubomir Dimitrov
Katya Milenova, Katerina
Zaharieva,
IrinaTechnology
Stambolova,
Blaskov,
PHOTOCATALYTIC PERFORMANCE OF TiO2, CeO2, ZnO AND TiO2-CeO2-ZnO
IN THE COURSE OF METHYL ORANGE DYE DEGRADATION
Katya Milenova1, Katerina Zaharieva1, Irina Stambolova2,
Vladimir Blaskov2, Alexander Eliyas1, Ljubomir Dimitrov3
Institute of Catalysis, Bulgarian Academy of Sciences
Acad. G. Bonchev St., bl. 11, 1113 Sofia
E-mail: [email protected]
2
Institute of General and Inorganic Chemistry
Bulgarian Academy of Sciences
Acad. G. Bonchev St., bl.11, 1113 Sofia, Bulgaria
3
Institute of Mineralogy and Crystallography
“Аcad. I. Kostov”, Bulgarian Academy of Sciences
Acad. G. Bonchev St., bl.107, 1113 Sofia, Bulgaria
1
Received 04 July 2016
Accepted 20 October 2016
ABSTRACT
Commercial TiO2, CeO2, ZnO single oxides and a mixture of them at a mass ratio of 1:1:1 were mechanochemically
treated (MCT). CeO2 was the best commercial sample prior to MCT because of its narrow band gap and high oxygen storage
capacity. The powder X-ray diffraction analysis confirmed the existence of TiO2, CeO2 and ZnO phases in all MCT samples
studied. The determined average crystallite size was in the range of 18 - 19 nm for the single component photocatalysts
and of 10 - 15 nm for the ternary system. Following 30 min dark period the adsorption capacity of TiO2-CeO2-ZnO-MCT
(0.263 mg/g) was the highest among those of the other materials. The behavior of the commercial and MCT photocatalytic
materials (TiO2, CeO2, ZnO and TiO2-CeO2-ZnO) was studied in the course of photocatalytic oxidative degradation of
Methyl Orange dye treated as a pollutant. The results obtained in an aqueous solution under UV irradiation showed that
ZnO-MCT sample provided the highest photocatalytic effect (81 % conversion) when compared to that of the other MCT
materials. The degree of MO dye degradation decreased in the line: CeO2 (87 %) > ZnO-MCT (81 %) > ZnO (76 %) >
CeO2-MCT (67 %) > TiO2-CeO2-ZnO-MCT (63 %) > TiO2 (61 %) > TiO2-MCT (49 %).
Keywords: Methyl Orange, dye, TiO2, CeO2, ZnO.
INTRODUCTION
Azo dyes comprise approximately one-half of all
textile dyes used today. A significant amount of the total
world production of these dyes is estimated to be released
into the waterways without complete decontamination.
The effluent waters discharged from different industries
such as textile, leather tanning, paper, and plastics are
usually polluted by such dyes [1, 2]. Photocatalysis has
been proposed as a green technology because of solar
energy utilization and high efficiency in degradation of
organic pollutants [3, 4]. Methyl Orange is an appropri-
ate model pollutant due to its solubility and presence in
wastewaters, which are object of strict environmental
regulations on discharging industrial effluents [5].
A photocatalyst is considered efficient, if it can
facilitate the competition of different interphase
processes involving e - and h + reactions with the
species adsorbed, decreasing thus the electron-hole
recombination degree [6]. Semiconductors activation
is an exciting strategy for photocatalytic degradation
of pollutants in an aqueous medium [4]. TiO2, ZnO,
CeO2, etc. are large band gap semiconducting materials
suitable as photocatalysts under UV-light irradiation
13
Journal of Chemical Technology and Metallurgy, 52, 1, 2017
for non-selective mineralization of several mutagenic
and carcinogenic hazardous organic pollutants present
in industrial waste water effluents [7, 8]. Photocatalytic
degradation of Reactive Red and Direct Green dyes on
commercial TiO2 Degussa P25, anatase TiO2 are investigated [9]. ZnO [5, 10] and CeO2 [11, 12] catalysts
are used to degrade photocatalytically Methyl Orange.
Some authors report that the combination of large band
gap semiconductors like TiO2 with other narrow band
gap semiconductors or post synthesis modification can
enhance dye degradation [13]. TiO2 and CeO2 - doped
TiO2 are evaluated for their adsorption performance in
Congo Red dye removal in an aqueous solution [14].
The degradation of Methyl Orange is studied on CeO2/
TiO2 under UV light [15], while on TiO2-CeO2 catalysts
under UV and sunlight irradiation [4]. Some CeO2/TiO2
nano-belt heterostructures [16] as well as CeO2-TiO2
films [17] are investigated under either UV or visible
light irradiation [16]. Ag-AgCl/CeO2 nanocomposite
plasmonic photocatalyst exhibit a high visible-light
photocatalytic activity in respect to photocatalytic
degradation of Methyl Orange in water because of
the synergistic effects brought about by Ag and AgCl
[18]. Fe2O3-CeO2-TiO2/Al2O3 is tested as a catalyst for
catalytic wet air oxidation of MO [19]. TiO2/SnO2 and
inverted core-shell SnO2/TiO2 nanocomposites are investigated under visible light in regard to their induced
photocatalytic activity in course of MO dye degradation. The photocatalytic performance of both core-shell
structures is vastly improved compared to that of bare
TiO2 and SnO2 nanostructures [20]. Xu et al. [21] study
the microwave catalytic degradation of MO in an aqueous solution over CuO/CeO2 catalyst in the absence and
presence of H2O2. A synergistic rather than an additive
effect of the catalyst, the microwave irradiation, and
H2O2 contribute to the high MO degradation activity.
The photocatalytic performance of SO42-/CeO2-TiO2/
HTLC photocatalyst is determined. TiO2 doping by
CeO2 enhances the catalytic activity of TiO2 in the visible light region. SO42- and CeO2-TiO2 show a synergistic
photocatalytic action [22]. Photocatalytic degradation
processes under visible irradiation are investigated in
the course of MO aqueous solution discoloration using undoped CeO2 and Fe doped CeO2 films. An iron
14
doping could be effective in preventing electron–hole
recombination, thereby increasing the lifetime of the
electron–hole pair separation [23].
The paper reports a comparative investigation of
the photocatalytic properties of commercial powders
of TiO2, CeO2, ZnO and mechanochemically treated
samples of TiO2, CeO2, ZnO and TiO2-CeO2-ZnO (at a
weight ratio 1:1:1) in the degradation under UV-light
irradiation of aqueous solutions of Methyl Orange
(MO) dye selected as a model contaminant. The phase
composition of the mechanochemically treated materials
is further studied by powder X-ray diffraction analysis.
EXPERIMENTAL
Mechanochemical treatment
The commercial powders of TiO2, CeO2 (Alfa Aesar
GmbH & Co KG) and ZnO (Chemapol) and a mixture
of TiO2, CeO2 and ZnO at mass ratio of 1:1:1 were
mechanochemically treated in an agate milling container
of 80 ml volume using a high-energy planetary ball mill
of PM 100 type (Retsch, Germany). The process was
carried out at a milling speed of 400 rpm, a milling time
interval of 10 min and a mass ratio of balls to powder
of 9:1 in an air medium.
Physicochemical characterization
The physicochemical characterization of the samples was performed by powder X-ray diffraction analysis
(PXRD). The PXRD patterns of the mechanochemically
treated samples were collected on a Bruker D2 Phaser
diffractometer within the range of 2θ values between 5º
and 55º using CuKα radiation (λ = 0.154056 nm) at 40 kV.
The phases present were identified by JCPDS database
(Powder Diffraction Files, Joint Committee on Powder
Diffraction Standards, Philadelphia PA, USA, 1997).
Photocatalytic activity testing
The photocatalytic oxidative degradation of MO
dye in an aqueous solution was investigated under UV-A
polychromatic irradiation (18 W with maximal emission
at 365 nm) and illumination intensity of 0.66 mW/cm2
using commercial TiO2, CeO2, ZnO and mechanochemically treated TiO2, CeO2, ZnO and TiO2-CeO2-ZnO
Katya Milenova, Katerina Zaharieva, Irina Stambolova, Vladimir Blaskov, Alexander Eliyas, Ljubomir Dimitrov
samples as photocatalysts. The starting concentration
of the tested MO dye aqueous solutions (150 ml) was
equal to 8 ppm. It was identical for all samples studied.
The UV-1600PC Spectrophotometer was used in the
wavelength range from 200 nm to 800 nm to measure the absorbance changes during the photocatalytic
experiments based on a previous calibration using MO
solutions of known concentrations. The investigated
reaction system was left to reach an equilibrium state
in dark for about 30 min (after mixing the solution and
photocatalyst sample in a slurry) prior to switching on
the UV illumination. The adsorption capacities of the
materials investigated was followed as a function of
the dark period duration. The evaluation referred to the
decreased absorbance of the suspension centrifuged to
obtain clear solution. 0.15 g of the photocatalyst was
used for the standard reference catalytic test (1mg/ml).
A semi-batch slurry photocatalytic reactor was used. It
fed air flow continuously. All photocatalytic measurements were carried out at a constant stirring rate (400
rpm) under ambient conditions. Aliquot parts of the
suspension were taken out of the reaction vessel at after
regular time intervals. The separation of the powder
from the suspension was carried out by centrifugation
prior to the UV-Vis spectrophotometrical absorbance
measurements. Thereafter, the separated amount was
returned back to the sampling solution, which ensured
operation under conditions of a constant volume and a
constant catalyst amount.
Fig. 1. PXRD patterns of commercial TiO2, CeO2 and
ZnO.
RESULTS AND DISCUSSION
Figs. 1 and 2 present the powder X-ray diffraction
(PXRD) patterns of the commercial and mechanochemically treated TiO2, CeO2, ZnO, TiO2-CeO2-ZnO
samples. TiO2 (PDF-21-1272), CeO2 (PDF-81-0792)
and ZnO (PDF-36-1451) single phases are registered in
the PXRD spectra of the commercial powders and the
mechanochemically treated materials. The purpose of the
mechanochemical treatment is to activate the samples
through crystallite sizes decrease and increase of the
specific surface area and surface defects number. The
obtained TiO2-CeO2-ZnO sample contains TiO2 (PDF21-1272), CeO2 (PDF-81-0792) and ZnO (PDF-36-
Fig. 2. PXRD patterns of mechanochemically treated
TiO2, CeO2, ZnO and TiO2-CeO2-ZnO photocatalysts.
15
Journal of Chemical Technology and Metallurgy, 52, 1, 2017
1451) phases. Table 1 lists the average crystallite sizes,
lattice microstrain parameters and unit cell parameters
of mechanochemically treated samples determined by
PowderCell 2.4 program [24]. As evident the calculated
mean crystallite size of the materials (which originally is
quite different) subjected to mechanochemical treatment
of 10 min reaches a value of 18 nm - 19 nm for the single
component photocatalysts, and of 10 nm - 15 nm for the
ternary system. Such short time of mechanochemical
treatment practically changes neither the parameter of
the crystal lattice, nor the degree of crystallinity – the
only substantial difference refers to the decrease of the
crystallites size.
Fig. 3 shows the adsorption capacities of the tested
TiO2, CeO2 and ZnO samples measured at regular time
intervals in course of the dark period. The adsorption
capacities are determined using the equation:
where C0 and C are the initial and current concentrations,
correspondingly, V is the volume of the solution, while
m is the mass of the catalyst.
The adsorption capacities of the commercial photocatalysts in respect to MO dye after 30 min contact time
without illumination follow the line: TiO2 (0.093 mg/g) <
ZnO (0.195 mg/g) < CeO2 (0.256 mg/g). Fig. 4 illustrates
the adsorption capacities of the tested mechanochemi-
cally treated TiO2, CeO2, ZnO and TiO2-CeO2-ZnO. The
values referring to 30 min adsorption in the dark are as
follows: CeO2-MCT (0.048 mg/g) < TiO2-MCT (0.104
mg/g) < ZnO-MCT (0.123 mg/g) < TiO2-CeO2-ZnOMCT (0.263 mg/g). CeO2-MCT shows the lowest adsorption capacity among all the samples studied. In fact,
the latter almost reach MO dye adsorption-desorption
equilibrium within about 10 min. For a comparison, Yu
et al. achieved azo dyes equilibrium adsorption on TiO2
(P25) photocatalyst [2] in 6 min.
The time dependence of the oxidative degradation
of MO in an aqueous solution calculated as [(C0-C)/C0]
x100, % is illustrated in Fig. 5. The evaluation is carried out under UV-A illumination at the wavelength (464
nm) of the diazo bond (-N=N-) absorbance maximum.
The fastest degradation of the dye is observed in presence of CeO2 (87 %), when compared to those of ZnO
(76 %) and TiO2 (61 %). The adsorption capacities are
in correspondence with the catalytic activities of the
samples investigated. It is worth noting that the greatest adsorption capacity and the highest degree of MO
degradation is obtained in presence of CeO2 among the
non-treated samples.
Fig. 6 compares the performance of the mechanochemically treated TiO2, CeO2, ZnO and TiO2-CeO2ZnO photocatalysts in MO degradation in an aqueous
solution. The calculations are carried out under the
conditions pointed above. In this case, the adsorption
Fig. 3. Adsorption capacities of tested TiO2, CeO2 and
ZnO during the dark period.
Fig. 4. Adsorption capacities of tested mechanochemically treated TiO2, CeO2, ZnO and TiO2-CeO2-ZnO during the dark period.
Q=
16
(C0 − C ) V
m
(1)
Katya Milenova, Katerina Zaharieva, Irina Stambolova, Vladimir Blaskov, Alexander Eliyas, Ljubomir Dimitrov
Table 1. Mean crystallite size (D), lattice strain (ε) and unit cell parameter
(a) of commercial and mechanochemically treated TiO2, CeO2, ZnO, TiO2CeO2-ZnO photocatalysts.
Sample
TiO2
TiO2, MCT
CeO2
CeO2, MCT
ZnO
ZnO, MCT
TiO2-CeO2-ZnO, MCT (1:1:1)
Fig. 5. The degradation of the MO dye in water
solution calculated as [(C0-C)/C0]x100, % with the
course of time under UV-A illumination at 464 nm
absorbance maximum, attributed to the peak of the
diazo bond (-N=N-) using investigated TiO2, CeO2
and ZnO photocatalysts.
Phase D, nm ε, a.u
a, Å
TiO2
22
2.2×10-3 3.782
TiO2
18
2.9×10-3 3.785
CeO2
39
1.3×10-3 5.408
CeO2
19
2.9×10-3 5.412
ZnO
22
2.5×10-3 3.250
ZnO
19
2.8×10-3 3.250
TiO2
10
3.4×10-3 3.775
CeO2
13
3.2×10-3 5.409
ZnO
15
3.1×10-3 3.246
Fig. 6. The degradation of the MO dye in water solution calculated as [(C0-C)/C0]x100, % with the course
of time under UV-A illumination at 464 nm absorbance
maximum, attributed to the peak of the diazo bond
(-N=N-) using investigated mechanochemically treated
TiO2, CeO2, ZnO and TiO2-CeO2-ZnO photocatalysts.
17
Journal of Chemical Technology and Metallurgy, 52, 1, 2017
capacity increase does not correlate with the catalytic
activity observed. This is probably due to the influence
of another factor – the surface polarity counterplay with
the adsorption capacity. The degrees of MO degradation
line as follows: TiO2-MCT (49 %) < TiO2-CeO2-ZnOMCT (63 %) < CeO2-MCT (67 %) < ZnO-MCT (81
%). It is assumed that the polar end of MO molecule is
irreversibly adsorbed on TiO2 causing thus deactivation.
This can explain the plateau observed in the curve of
MO degradation on TiO2 (Fig.6). The nonpolar surfaces
of CeO2 and ZnO [25] exclude such behavior.
The oxidative decomposition of the model pollutant Methyl Orange is connected with irradiation of the
photocatalyst by high-energy UV photons. They excite
the photoelectrons in the conduction band, which in
turn are consumed by the adsorbed O2 molecules. Thus
the lifetime of the holes is prolonged due to this charge
separation, which migrate to the surface and forms a
hydroxyl radical and preventing recombination [5].
Lower photocatalytic degradation rates are observed
on the surfaces of TiO2 and ZnO due to their wide band
gaps (respectively 3.2 eV and 3.37 eV) and fast recombination rate of photogenerated electron–hole pairs. The
highest efficiency is manifested by a narrow band gap
metal oxide (CeO2 Eg = 2.54 eV) activated by both UV
and visible light. Although only UV light is used in the
present experiments, this material’s behavior is attributed
to the additional amount of hydroxyl radicals generated
[4] and its great oxygen storage capacity.
CONCLUSIONS
The photocatalytic study performed shows that the
commercial sample of CeO2 displayed the highest degree
of degradation of MO dye (87 %) compared to those of
the other photocatalysts investigated probably because of
its ability to generate an additional amount of hydroxyl
radicals and high oxygen storage capacity. MCT promoting effect is observed only with ZnO (81 % degree of
MO degradation). The mechanochemically treated TiO2
sample exhibits a lower photocatalytic activity than that
of the commercial powders. The negative effect in this
case is attributed to the surface polarity bringing about
deactivation of the active sites through irreversible ad-
18
sorption of the polar dye molecules. All samples studied
demonstrate good photocatalytic activity in MO degradation and they could possibly be applied in this dye
removal from wastewaters under UV-illumination. The
highest adsorption capacity observed following 30 min
dark period is shown by the mechanochemically treated
TiO2-CeO2-ZnO (0.263 mg/g), when compared to those
of the other samples: CeO2 (0.256 mg/g), ZnO (0.195
mg/g), ZnO-MCT (0.123 mg/g), TiO2-MCT (0.104
mg/g), TiO2 (0.093 mg/g) and CeO2-MCT (0.048 mg/g).
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