Effect of Dye Type on MMT-Supported Pr-Doped TiO2 Composite Photocatalyst

Vol. 130 (2016) ACTA PHYSICA POLONICA A No. 1
Special issue of the 2nd International Conference on Computational and Experimental Science and Engineering (ICCESEN 2015)
Effect of Dye Type on Montmorillonite-Supported
Pr-Doped TiO2 Composite Photocatalsyt
B. Otsukarci and Y. Kalpakli∗
Yildiz Technical University, Department of Chemical Engineering, 34210, Istanbul, Turkey
Scarcity of water in today’s world has led to scientific researches for finding smarter ways of discharging and
re-using water located in the seas and lakes. That is how advanced oxidation technologies have emerged as one of
the new discharging techniques to keep water sources clean. In our method we have synthesized praseodymium (Pr)
doped TiO2-Montmorillonite composite photocatalyst by using acid sol-gel method. Pr, in the group of rare earth
metals, is known for its ability to form Lewis bases in order to break down aldehydes, amines, etc., by decreasing
the crystal size and consequently increasing the surface area. We have used the mineral form of the bentonite clay
(montmorillonite) to increase the surface area of our composite photocatalyst. Two types of azo dyes which are
commercially known as Basic Yellow 28 (BY 28) and Basic Blue 41 (BB 41) were used in experimants. These dyes
are composed of different chemical structures: BY 28 is known as azomethine dye (–CH=N–), whereas BB 41 has
one azo bond (–N=N–). The initial model dye concentrations were 100 ppm. Amount of 1.0 g/l of photocatalyst
was used throughout the experiments. In order to evaluate degradation efficiency results, the dissolved organic
carbon analyzer was used. We have found the optimum dark adsorption time to be under 15 minutes. By stirring
under 15 minutes in the dark we have achieved efficiency of 52.50% for BY 28 and 66.74% for BB 41. On the other
hand, by stirring under UV-A light irradiation for two hours we have achieved the overall degradation efficiency
of 86.30% for BY 28 and 92.39% for BB 41. Our results show that higher adsorption and overall efficiencies were
observed in BB 41 than in BY 28. Moreover, according to Water Pollution Control Regulations, the chemical oxygen
demand limitations for textile industry are between 200–400 mg/l. To evaluate the degradation characteristics of
Pr-doped TiO2-Montmorillonite composite, the change of chemical oxygen demand values with time for 100 mg/l
of BY 28 and BB 41 were investigated. The obtained chemical oxygen demand values were 60.5 and 34.8 mg O2/l
after 2 hours of oxidation processes for BY 28 and BB 41. As it can be seen from the results, obtained values are
below the limitations.
DOI: 10.12693/APhysPolA.130.198
PACS/topics: 81.16.Hc, 87.64.K, 87.64.km
1. Introduction
Recently, heterogeneous photocatalysis of TiO2 doped
with rare earth elements has attracted extensive atten-
tion due to broadening of the oxidation activity of TiO2
into the visible region of spectrum, which decreases the
cost of degradation and makes the process feasible for in-
dustrial usage. Even though TiO2 has a large band gap
(3.2 eV in the anatase form) and strong oxidation po-
tential of the photogenerated holes (∼ 2.9 V vs NHE at
pH 0), its efficiency is limited under the visible light [1].
Photocatalytic reaction has several steps: (a) produc-
tion of electron-hole pairs (e−
/h+
) by generating the re-
quired over-band-gap energy on the photocatalyst sur-
face, (b) separation of e−
/h+
pairs by electron scavengers
or traps on TiO2 surface, (c) a redox process between
the separated e−
and h+
and the pollutants located in
the solution, (d) desorption of the products from the
surface [2].
One of the ways to extend the visible light sensitiv-
ity spectrum of TiO2 from UV area is to prolong the
e−
/h+
separation in order to have effective redox pro-
cesses. Here is where the incorporation with rare earth
∗corresponding author; e-mail: kalpakli@yildiz.edu.tr
elements becomes useful. Praseodymium, which is an el-
ement belonging to rare earth elements group, was used
in this study as a doping agent. According to Xiuqin
et al. doping of the photocatalyst with the appropriate
amounts of rare earths improves the photocatalytic ac-
tivity of the photocatalyst by increasing the formation
rate of e−
/h+
pairs and thus increasing the amount of
hydroxyl radicals (·OH) [3]. Su et al. have stated that
doping with Pr induces the photocatalytic activity in vis-
ible range of spectrum [4].
In this study our aim was to prepare TiO2-
Montmorillonite (MMT) doped with Pr, using acid sol-
gel method, in order to compare the degradation and de-
coloration results for two model pollutants. As the model
pollutants, two colorants, Basic Yellow 28 (BY 28) and
Basic Blue 41 (BB 41), were used. MMT, which is a min-
eral form of bentonite clay, widely found in Turkey, was
used as the supporting material for TiO2. Both, BY 28
and BB 41 are basic, cationic dyes soluble in water due
to their sulfonate groups (SO−
3 ) [5]. BY 28 is also known
as the azomethine dye (-CH=N-) or the hydrazone dye
(=N-N(H,R)-) [6]. BB 41 is a mono azo dye, due to pres-
ence in its structure of one azo group (-N=N-) [7]. BY 28
is used in acrylic, wool, and nylon dyeing [8]. BB 41 is
suitable for acrylic, wool, and cotton dyeing [9].
(198)

Effect of Dye Type on Montmorillonite-Supported Pr-Doped TiO2 Composite Photocatalsyt 199
2. Experimental
BY 28 and BB 41 were purchased from Alptekim Dye
and Chemicals Incorporated Company. The commercial
name of BY 28 is Acryla Golden Yellow GL and the com-
mercial name of BB 41 is Acryla Blue GRL. Pr-doped
TiO2-MMT has been synthesized using analytical grade
precursors. These precursors were tetra-tert-buthyl-
orthotitanate (Merck), absolute ethanol (Merck), nitric
acid (Merck) and sodium bentonite (Esan Eczacıbaşı
Industrial Raw Materials Incorporated Company, Kü-
tahya). Praseodymium (III) nitrate hexahydrate 99.9%
with a chemical formula of Pr(NO3)3 · 6H2O was pur-
chased from Sigma-Aldrich. Distilled water has been
used in all experiments.
2.1. Equipment
All adsorption and photocatalytic oxidation experi-
ments were conducted using Luzchem photoreactor. This
reactor was equipped with eight UV-A black fluorescent
8 W lamps. Lamps were turned off during the adsorption
experiments. WiseShake branded stirrer was used in the
synthesis step. Dissolved organic carbon was analyzed
using Hach-Lange IL 550 TOC-TN analyzer. In order to
determine the effect of photocatalyst on the color prop-
erties of water, absorbance values after each experiment
were measured using Perkin Elmer branded Lambda 35
UV/VIS spectrometer at λ = 415 nm for BY 28 and at
λ = 617 nm for BB 41.
2.2. Experimental procedure
For the experiments we have employed solutions of
25 ml of BY 28 and BB 41 in a 100 ml beaker. Concentra-
tion was kept at 100 ppm throughout the experiments for
both dyes. Optimum adsorption and degradation data
was obtained step by step.
2.3. Adsorption process
As known from the previous experiments and literature
findings, photocatalytic reactions take place collabora-
tively in two stages: adsorption and degradation. In the
adsorption stage we have determined the optimum stir-
ring conditions in the dark, when the UV lights were
turned off, by trying stirring of BY 28 for 15, 25, 35, and
45 minutes.
2.4. Photocatalytic reactions
Degradation process was studied when we have deter-
mined our optimum adsorption time. Our aim was to
obtain the lowest adsorption percentage and the highest
oxidation potential. Our optimum adsorption time for
BY 28 was 15 min. After that we have tried out dif-
ferent photocatalyst amounts of 1.0, 1.5, 2.0, 2.5, and
3.0 g/l under UV-A irradiation time of 35, 45, 60, 90,
and 120 min. We have obtained the highest oxidation
of BY 28, which had settled at around 120 min, using
1.0 g/l of photocatalyst. Later we have compared our re-
sults with those obtained for BB 41. In the experiments
with BB 41 the degradation stage had lasted 30, 60, 90,
and 120 min.
3. Results and discussion
3.1. Photocatalytic degradation experiments
Adsorption is an important step in the studied pro-
cess, which increases the amount of compounds to be de-
graded under the UV-A irradiation. We have determined
adsorption efficiencies for both dyes. After 15 min of ab-
sorption, the adsorption efficiency was 52.50% for BY 28
and 66.74% for BB 41. BB 41 had a higher adsorption ca-
pacity compared to BY 28. The highest overall efficiency
(adsorption% + oxidation%, after 120 min of UV-A ir-
radiation) was 86.30% for BY 28, which is less than that
for BB 41, with a value of 92.39%.
When the UV-A irradiation time was increased from
30 to 120 min, the surface attacking rate for both dyes
had decreased and finally had stabilized around the same
value, as shown in Fig. 1. However, the initial attacking
rate of BB 41 to the surface of the photocatalyst was
higher. According to Bouafia et al. [10], who have applied
the photo-Fenton process, the rate of oxidation of BB 41
was also higher than that of BY 28, which is coherent
with our findings.
Fig. 1. Surface attack rate for BY 28 and BB 41 as
function of UV-A irradiation time, after 15 min of dark
adsorption. Concentration of the model compounds was
100 ppm. Amount of the photocatalyst used was 1.0g/l.
Kinetic results obtained by varying the irradiation time
have shown that the 1st order Langmuir-Hinshelwood ki-
netics is applicable under these steady conditions. In this
manner, values of the kapparent and t1/2 for BY 28 were
were 0.0094 min−1
and 101.9 min, as shown in Table I.
Even though photocatalyst attacked BB 41 faster due
to higher adsorption yield reaction proceeded slower to
reach its half-life value compared to BY 28.
Giwa et al. found apparent rate constant for 100 ppm
of BB 41 as 0.005 l/mol min using only TiO2 Degussa
P-25 as a photocatalyst after 60 min of irradiation [11].
Our results are in a good agreement with the literature
result.

200 B. Otsukarci, Y. Kalpakli
TABLE I
Calculated apparent rate constant and half life values for
BY 28 and BB 41 dyes.
kapparent [min−1
] kapparent [l/mol min] t1/2 [min]
BY 28 0.0068 0.0085 73.4
BB 41 0.0094 0.0055 101.9
After the degradation period, decolorization efficiency
of dyes was observed as a function of time using a spec-
trophotometer.
3.2. UV-Vis spectroscopy
Degradation study of BY 28 and BB 41 was followed
by a study using UV-Vis spectroscopy. The results are
depicted in Fig. 2. Discoloration efficiency (%) for the
BB 41 was lower than that of BY 28 at the end of 2 h
of irradiation. However, at the end of 90 min period of
irradiation BB 41 had reached its highest discoloration
efficiency. Therefore, the highest discoloration efficiencies
calculated at the end of 2 h for BY 28 and 90 min for
BB 41 were 84.05% and 80.94%, respectively. This is due
to the different chemical structures of the dyes.
As the overall efficiency had increased, the absorbance
values for discoloration of the dyes had eventually de-
creased after 120 min of illumination, as shown in Fig. 2.
In Fig. 2a TOC removal efficiency has a sigmoidal shape,
indicating the formation of by-products that slow down
the degradation efficiency and the degradation rate cal-
culated in Table I for BY 28 [12].
Fig. 2. Comparison of the change of the TOC removal
(blue) and the remaining dye absorbance value (red)
with the irradiation time, for two model dyes (a) BY 28
and (b) BB 41.
3.3. FTIR
At the end of the study, the characteristic peaks of
BY 28 and BB 41 were investigated using FTIR analysis.
For BY 28 these characteristic peaks were found at 1638,
1604, 1552, 1328, 1178, 935, 829, 786, and 642 cm−1
, as
shown in Fig. 3. In an orderly fashion peak at 1638 cm−1
is related to stretching of the C=N group, due to hy-
drazine form of the dye [13], peak at 1604 cm−1
is re-
lated to vibration of the C=N group [14], 1552 cm−1
is
related to combined vibration of NH–N=C group’s N–H
bending and –N=C stress [15], peaks from the range of
1550–1300 cm−1
, 1463, and 1330 cm−1
belong to the
alkanes and alkenes [16], 1247 cm−1
from the range of
1300–1000 cm−1
belongs to the ethers [16], 1178, 1120,
and 1034 cm−1
peaks belong to the S=O stress due to
RO–SO−
3 [16], 935 and 829c m−1
peaks belong to the
C–H bending vibrations, located in the range of 950–
780 cm−1
[17, 18].
Fig. 3. Comparision of FTIR results for BY 28. (a)
BY 28 dye powder, (b) 15 min adsorption, (c) 15 min ad-
sorption + oxidation (d) 1% Pr, 25% TiO2-MMT com-
posite photocatalyst.
Range 1300–900 cm−1
in the spectrum belongs to the
aromatics group in the composition of the dye, which is
removed from the solution with the sequential adsorp-
tion and degradation reactions. In addition, peaks at
935 and 642 cm−1
also disappear. A characteristic peak
at 1604 cm−1
was also cleared away at the end of redox
reactions.
Peak at 3610 cm−1
, which is the characteristic peak of
the photocatalyst, due to OH stretching reduced BY 28
dye powder’s 3610 cm−1
peak represented by the C–H
bending.
Characteristic peaks of mono azo dye BB 41 were also
analyzed. For BB 41 these characteristic peaks are lo-
cated at 1610, 1547, 1483, 1426, 1402, 1372, 1328, 1268,
1110, 984, 860, 829, and 732 cm−1
, as shown in Fig. 4.
Peak at 1610 cm−1
belongs to the amines and amides
represented by the range of 1650–1550 cm−1
[16], peaks
at 1547, 1483, 1426, and 1402 cm−1
are related with
the N=N vibration, represented by the 1500–1400 cm−1
range [19], peaks at 1372, 1328, and 1268 cm−1
are due
to symmetric C–O stretching [14], 1114 cm−1
is related
to stretching of S=O, due to RO–SO−
3 group [16], peak
at 987 cm−1
belongs to the alkenes and aromatics in the

Effect of Dye Type on Montmorillonite-Supported Pr-Doped TiO2 Composite Photocatalsyt 201
range of 1000–650 cm−1
[16], peaks at 860 and 829 cm−1
from the range of 950–780 cm−1
are related with the C–H
bending [19] and to the range of 800–400 cm−1
, due to
aromatic compounds [16].
Fig. 4. Comparision of FTIR results for BB 41. (a)
BB 41 dye powder, (b) 15 min adsorption, (c) 15 min ad-
sorption + oxidation (d) 1% Pr, 25% TiO2-MMT com-
posite photocatalyst.
When 1300–950 cm−1
range is investigated, it is seen
that, due to sequential reactions of adsorption and ox-
idation, the aromatic compounds, peaks of which are
located in this area, are removed. Therefore, peaks at
1166, 1114, and 987 cm−1
are reduced at the end of re-
actions. In the range of 1700–1270 cm−1
peaks at 1483,
1402, and 1372 cm−1
, resulted from C=N bond due to
aromatic compounds, amines, alkanes, and alkenes, are
also degraded. Peaks at 860, 829, 732, 619 cm−1
are
also removed by the sequential adsorption and oxidation
reactions.
4. Conclusions
In this study degradation and decoloration of two basic
cationic azo dyes were investigated as a function of irra-
diation time. This work demonstrates for the first time
that MMT-supported Pr-doped TiO2 composite photo-
catalyst has a high efficiency for both degradation and
decoloration. The degradation and decoloration rates
of the dyes and their efficiencies were influenced by the
time the dyes were exposed to the UV-A light. High-
est TOC removal achieved for BY 28 and BB 41 were
86.30% and 92.39% at 120 min respectively. Whereas
the most discoloration efficiency was achieved at the end
of 2 h with 84.05% for BY 28, the highest discoloration
rate for BB 41 was achieved at the end of 90 min with
80.94%. Even though, production of by-products in the
degradation reaction of BY 28 is observed, the system
is fast enough to overcome inhibition of oxidation and
continues the photocatalytic reaction.
FTIR analysis helped us to determine the character-
istic peaks of BY 28 and BB 41 and had shown that
most of the dye removal is achieved only after 2 h of ir-
radiation. FTIR analysis results supported the results of
TOC and UV-Vis analysis. The most important part of
this report is the conduction of the experiments without
using oxidation agents. In the further experiments oxi-
dation agents will be used to improve degradation and
discoloration efficiencies in a shorter irradiation time.
Acknowledgments
The authors would like to thank Yıldız Technical Uni-
versity BAPK project no. 2014-07-01-YL05 for financial
support of this work.
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