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Transcript of 12) CHAPTER 4 (rehwtvised)-119-162
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CHAPTER 4
RESULTS AND DISCISSION
C coated TiO2-P25
4.1 Physical characterization
4.1.1 Elemental, BET and HR-TEM analyses
TiO2 Degussa P25 which is made up of 80 and 20% of anatase and rutile [144]
respectively, is a commercially available and relatively cheap TiO2 powder used for
many industrial applications. For that reason, this kind of TiO2 was applied for the next
modifications towards enhancing their photocatalytic activity. Therefore, the word
TiO2-P25 will be representing TiO2 for the entire Chapters 4 and 5. Table 4.1
(columns 4 and 5) summarized the C and N contents of the photocatalyst samples
prepared at various heating temperatures and dosages of peat. Pristine TiO2-P25 sample
was also heated at 450 C for control purposes. The control sample showed zero
amounts of C and N content.
As shown in Table 4.1, it was found that the amount of carbon and nitrogen
content increased with increasing amount of peat loading, but decreased with the
increasing heating temperature. The carbon and nitrogen content decreased by about
38% and 27% respectively when the temperature was increased from 400 to 600 C. The
decreasing amount of carbon and nitrogen is due to the sintering effect of carbon and
nitrogen at high temperatures. Similar finding was also reported by Tsumura et al. [145]
The amount of elemental nitrogen content was small for all treated samples but
definitively proved that nitrogen was present in the treated TiO2.
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Table 4.1: Experimental conditions and results of some characterizations of the C coated
TiO2-P25. Pseudo first order rate constants (k) for the degradation of 30 mg L-1
RR4anionic dye were obtained under low energy 45 Watts fluorescent light irradiation with
residual UV leakage of 3.5 W m-2
.
TiO2 Temp. Peat C N SBET Rate const. Eg.
samples (C) (g) (wt%) (m2
g-1
) (k, min-1
) (eV)
P25 - 0 0.000 0.000 49.25 0.075 3.1
P25 450 0 0.000 0.000 49.21 0.077
PP0.1-400 400 0.1 0.047 0.034 0.081
PP0.1-450 450 0.1 0.041 0.034 0.113 2.9
PP0.1-500 500 0.1 0.041 0.027 0.111
PP0.1-550 550 0.1 0.038 0.021 0.113
PP0.1-600 600 0.1 0.037 0.017 0.008
PP0.4-400 400 0.4 0.084 0.068 0.09
PP0.4-450 450 0.4 0.080 0.061 0.141
PP0.4-500 500 0.4 0.080 0.050 0.127
PP0.4-550 550 0.4 0.074 0.047 0.134
PP0.4-600 600 0.4 0.072 0.044 0.095
PP0.6-400 400 0.6 0.152 0.097 0.092
PP0.6-4501 450 0.6 0.122 0.090 57.17 0.145 2.8
PP0.6-500 500 0.6 0.117 0.084 0.127
PP0.6-550 550 0.6 0.100 0.078 0.137
PP0.6-600 600 0.6 0.094 0.070 0.097
PP0.9-400 400 0.9 0.174 0.126 0.081
PP0.9-450 450 0.9 0.172 0.121 0.133 2.7
PP0.9-500 500 0.9 0.167 0.118 0.092
PP0.9-550 550 0.9 0.164 0.118 0.109
PP0.9-600 600 0.9 0.162 0.115 0.087
1 Labeling of samples such as PP0.6-450 means TiO2-P25 coated with 0.6 g peatcalcined a 450 C to produce C coated TiO2-P25 samples. SBET: BET surface area,
Table 4.1 also shows the BET surface areas values for PP0.6-450 and pristine
TiO2-P25 samples. Interestingly, the BET surface area of PP0.6-450 was found to be
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higher as compared to the pristine TiO2-P25. The higher BET surface area of PP0.6-450
may be attributed to the effect of sonication because it could produce crack and porous
structure on the TiO2 particle. Sonication process does not only help in the homogeneous
distribution of the carbon precursor on the TiO2 particle but also increases the surface
area of the catalyst. The energy from the sonication is known to produce cavitation
process in liquid form. The cavitation is the process of bubbles production [146]. The
compression (positive pressure) and rarefaction (negative pressure) due to the rapid
agitation from ultrasonic waves caused the bubbles to collapse in compression, which
increased the temperature of the fluid [146]. The cavitation process from the
ultrasonication is believed to cause an increase in the surface area of TiO2, as mentioned
by Colmenares et al. [147]. They found that the surface area of their prepared modified
TiO2 increased by 200% and a great decreased in pore size was observed under
ultrasonic treatment process. It also reduced the aggregation of the TiO2 fine particles
and break up into smaller particle sizes, as seen in TEM image in Figure 4.1(b) and
4.1(c). HRTEM image in Figure 4.1(a) was used to estimate the thickness of C on the
surface of P25particles. The image shows that the particle of TiO2-P25 is surrounded
with one layer of C fringes (Figure 4.1a) with the spacing similar to the standard fringes
of graphite of about 0.34 nm [128-130]. Such a thin coating is expected to allow the
penetration of light to the surface of TiO2 and should yield enhanced photocatalytic
activity.
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4.1.2 X-ray diffraction (XRD) analysis
As provided in the earlier discussion in Section 3.1.1 a thin carbon coated on the
surface of TiO2-P25 could allow the penetration of light to the surface of TiO2-P25and
should be able to enhance its photocatalytic activity. Figure 4.2 shows the XRD patterns
of the pristine TiO2-P25 and C coated TiO2-P25 at different calcination process. No
phase transformation occurred during the calcinations process for all samples at
calcining temperatures between 400C to 600C.
As stated in Chapter 3, pp 75, Zhang et al. [70] observed that the phase
transformation in the presence of a C layer occurred only when the calcining
temperature was at 900 C while Hsu et al. [119] found that phase transformation for
TiO2 calcined without C precursor occurred at temperature 700 C onwards.
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Figure 4.2: XRD patterns of the pristine P25 and C coated TiO2-P25 samples preparedat 0.6 g peat coagulant dosage at different calcination temperatures.
PP0.6-600
PP0.6-500
PP0.6-450
PP0.6-400
P25
Lin(CPS)
2 Tetha-Scale
10 30 50 7010 20 40 60
2 (deg.)
A
AA
A
R
R
A
A
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4.1.3 UV-Vis Diffused Reflectance Spectra (UV-Vis DRS) analysis.
Figure 4.3(a) shows the diffuse reflectance spectra of C coated TiO2-P25 and
pristine P25. The pristine P25 exhibited absorption only in the UV region, whereas the
optical response of C coated TiO2-P25 has absorption bands both in the UV and visible
region.
The energy band gap can be determined by extrapolation plots of Kubelka-Munk
versus energy (eV) as seen in Figure 4.3(b). The bandgap energy for all C coated TiO2-
P25 were reduced to less than 3.0 eV (Table 4.1), indicating that C coated TiO 2-P25
underwent red shift into visible light region. This observation may be due to the
presence of nitrogen which introduced an impurity level between the valence and
conduction band of TiO2 [148] or narrow the band gap by mixing the N 2p and O 2p
states [47].
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a)
b)
Figure 4.3: UV-Visible diffused reflectance absorption spectra (a) and plots of the
transformed Kubelka-Munk function versus the energy of the light
absorbed (b) of pristine TiO2-P25and C coated TiO2-P25.
0
2
4
6
1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9
eV
4.12.9 3.73.3 4.92.2 4.52.5
(K*hv)1/2[eV1/2 ]
0
6
4
2
PP0.9-450
PP0.6-450
PP0.1-450
P25
3.0 eV
PP0.9-450
PP0.6-450
PP0.1-450
TiO2-P25
0
0.5
1.0
1.5
300 400 500 600
Wavelength (nm)
500400 600300
Absorbance
..
0
1.5
1.0
0.5
PP0.9-450
PP0.6-450
PP0.1-450
P25
PP0.9-450
PP0.6-450
PP0.1-450
TiO2-P25
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4.1.4 X-ray Photoelectron Spectroscopy (XPS) analysis
The chemical state and composition of the C coated TiO2-P25 at the surface were
studied by XPS in which the presence of Ti2p, O1s, N1s and C1s (Figure 4.4a, b, c and
d) were confirmed with the binding energy (main peaks) of 459.5, 531.5, 405.5 and
284.5eV respectively as can be seen in Table 4.2. As shown in Figure 4.4a, the XPS
spectrum for Ti2p shows binding energies at around 459.3 and 465 eV, implying the
presence of Ti 2p3/2 and Ti 2p1/2 respectively [121]. The deconvolution of O1s in Fig. 4b
represents O-H binding energy at 530.9 eV [122,149], while the small peak at 531.8 eV
is assigned to Ti-O bond [150,151]. The peak observed at 395.7 eV (see Fig. 4c) is
synonymous with doped N in the form of Ti-N as observed by many reports [152,153]
while that at 399.6 eV is also commonly associated for N-doped TiO2 for N-H bonding
and interstitial doping into the lattice [153,154]. Therefore XPS data clearly proved that
N was doped into TiO2 lattice even though the amount was quite small. The broad
strong peak at 404 eV is normally associated with N-O bond possibly in the form of
adsorbed NO2- or NO3- [155]. In Figure 4.4(d), the deconvolutions of C1s peaks were
observed at 284.6 eV which can be assigned as adventitious element of carbon from
carbon tape during the preparation of the sample [123,156], while the binding energy at
283.5 eV represents C-C bond which is attributed to the carbon graphite bonding [124].
This confirmed the observed HRTEM results earlier which indicated the presence of the
graphitic C layer. No peak was found around 282.6 to 282.9 eV which is normally
associated with Ti-C [126], thus the possibility of C substitution doping was not
observed in PP0.6-450. Therefore, from the XPS spectra analysis, the term carbon
coated TiO2-P25 is best to describe and represent the modified sample since the amount
of nitrogen was very low.
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a)
b)
12500
15000
17500
1020 1025 1030
Ti 2p
Binding energy (eV)
455465
8
12
4
x 10 3
CPS
460470
16
20
Ti 2p3/2
Ti 2p1/2
20000
30000
950 955 960 965
O 1s
Binding energy (eV)
528
15
20
20
x 10 3
CPS
532 524536
25
30
520538
O-H
Ti-O
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c)
d)
Figure 4.4: XPS spectra of (a) Ti2p, (b) O1s, (c) N1s and (d) C1s for the C coated
TiO2-P25(PP0.6-450).
11000
12000
13000
1070 1080 1090
N 1s
Binding energy (eV)
405 400 395410
120
130
110
x 10 2
CPS
Ti-N
415 490
Binding Energy (eV)
O-N
Ti-N
10000
12000
14000
16000
1195 1200 1205
C 1s
Binding energy (eV)
280290
130
140
x 10 2
CPS
285 275295
150
160
Binding Energy (eV)
Carbon Tape
Carbon Graphite
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Table 4.2: The binding energy values for each element in carbon coated TiO2-P25
(PP0.6-450) sample.
Binding Energy (eV)
Elements
Main peak Deconvolution peaks
Ti 2p 459.5 465.0, 459.3
O 1S 531.5 529.2, 530.9
N 1S 405.5 404.9, 395.7, 399.6
C 1S 284.5 284.5, 283.5
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4.1.5 Photoluminescence (PL) analysis
The PL emission spectra can be used to reveal the efficiency of charge carrier
trapping, immigration, transfer and to understand the fate of photo-induced electrons and
holes in a semiconductor [157, 158]. It is known that the PL spectrum is the result of the
recombination of excited electrons and holes where the lower PL intensity means the
lower recombination rate of e- andh+ under light irradiation [159]. Figure 4.5 indicates
the PL spectra of pristine TiO2-P25 and C coated TiO2-P25 samples. All C coated TiO2-
P25 have lower PL intensity as compared against pristine P25. According to Janus et al
[74], carbon can also serve as an electron scavenger which is capable to prevent
electron-hole recombination.
This indicates that the presence of carbon graphite deposited into TiO2-P25 can
reduce the recombination rate of photoinduced electrons and holes in C coated TiO 2-
P25, as suggested by its low PL intensity shown in Figure 4.5. Furthermore, PP0.6-450
possesses the lowest PL intensity and therefore would be expected to have the highest
photoactivity c.a. 0.146 min-1(Table 4.1).
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Figure 4.5: Photoluminescence spectra of pristine P25 and C coated TiO2-P25prepared at 450 C containing different amount of peat loading.
Wavelength (nm)
250 500 750 1000
PL
Intensity
200
400
600
800
1000
0
P25
PP0.4-450
PP0.9-450
PP0.6-450
Wavelength (nm)
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4.2 Optimization experiments of C coated TiO2-P25
4.2.1 Effect of carbon content and calcination temperature towards
photocatalytic degradation of RR4 dye
For optimization study in the preparation of C coated TiO2-P25, anionic RR4 dye
was used as a model pollutant. Figure 4.6 shows the first order rate constant k
degradation of RR4 for pristine and C coated TiO2-P25 at various amount of peat
coagulant loading and calcination temperature. It can be seen that the rate of RR4
removal for all C coated TiO2-P25 was higher than pristine TiO2-P25. As shown in
Figure 4.6, at every selected calcining temperature, the photocatalytic activity of the
products for the degradation of RR4 increased with increasing peat coagulant content up
to the optimum value of 0.6 g. By going beyond this value, the photocatalytic activity
for all products at applied calcining temperatures decreased. The decrease in the
photocatalytic activity beyond the 0.6 g of peat coagulant loading was due to the
increasing rate of electron-holerecombination as predicted by the PL signals (Figure
4.5).
Figure 4.6 also indicates that C coated TiO2-P25 at different calcining
temperatures also yielded different photocatalytic activities and this again followed the
trend of PL signals whereby pristine TiO2-P25 < PP0.4-450 < TC0.9-450< TC0.6-450
(Table 4.1). Therefore, PP0.6-450 sample is considered as the optimum product prepared
using the optimum amount of peat coagulant (0.6 g) and optimum calcination
temperature (450C). The optimum PP0.6-450 sample had a photocatalytic activity of
about 1.9 times faster than the pristine P25 under a 45 W compact fluorescent lamp light
source for the degradation of RR4.
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Figure 4.6: Pseudo first order rate constant for the degradation of 30 mg L-1
RR4 dyeunder different amount of peat modified TiO2-P25 represent by different
amount of peat at various calcinations temperature.
0.06
0.08
0.1
0.12
0.14
0 0.2 0.4 0.6 0.8 1 1.2
Amount of peat (wt%)
1storderrateconst.,k/min..
400
450
490
560
5901storderrateconst.(min-1)
Amount of peat (wt%)
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4.2.2 Effect of the sonication time in the preparation of C coated TiO2-P25
towards the photocatalytic degradation of RR4 dye
The homogenized mixture of TiO2-P25 and peat coagulant solution is very
important to produce the optimum photocatalytic activity of the calcined product. The
sonication process was found to be the best process to produce a homogenized mixture
of TiO2-P25 and peat. The effect of sonication time of prepared PP0.6-450 on the
photocatalytic activity under degradation of RR4 dye is represented in Figure 4.7.
Sample PP0.6-450 prepared under 8hr of stirring process was used as a comparison
study. The result shows that sample PP0.6-450 that was prepared by sonication process
has higher photocatalytic activity when compared against the product prepared via 8hr
stirring process.
Evidently, the pseudo first order rate constant increased with increasing
sonication time. The samples prepared with a longer sonication process exhibited higher
photocatalytic activity. Therefore, it can be concluded that the best formulation for the
preparation of the photocatalyst was obtained by eight hours of sonication which is
similar to the preparation of C coated TiO2 in Section 3.2.2. Therefore, the sonication
time for the preparation of all samples in this study was fixed at around eight hours.
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Figure 4.7: The effect of sonication time of the solution mixture on the photocatalyticactivity of the photocatalysts.
0.08
0.1
0.12
0.14
0.16
0 2 4 6 8 10 12
Times (h)
1st.orderrateconst.,k/mi
Time (h)
1stord
errateconst.(min-1)
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4.3 Photocatalytic evaluation of the C coated TiO2-P25 samples
4.3.1 Adsorption and photocatalytic degradation of RR4, MB and phenol by C
coated TiO2-P25 TiO2 samples
In this work, anionic RR4, MB and phenol were used to study the adsorption and
photocatalytic activity of C coated P25. Figure 4.8 shows the adsorption of RR4, MB
and phenol by pristine TiO2-P25 and C coated TiO2-P25 (PP0.6-450). As expected
PP0.6-450 has higher adsorption capacity compared with pristine TiO2-P25. The
adsorption for PP0.6-450 increased up to 9, 6 and 3% for RR4, MB and phenol
respectively by comparison with pristine TiO2-P25. An increased adsorption of RR4,
MB and phenol by PP0.6-450 was due to the increased surface area of the sample to
57.17 m2
g-1
(Table 4.1). Two processes occurred simultaneously under PP0.6-450 and
pristine TiO2-P25 namely adsorptive and photocatalytic processes. In order to isolate
the photocatalytic process from the adsorption part, a study was also done whereby the
samples were first presaturated with RR4 prior to switching on the light for
photocatalysis.
In this way, it was assumed that only photocatalytic process occurred in the
removal of pollutants. As shown in Figures 4.9 (a), (b) and (c), the RR4, MB and
phenol removal was always better when both processes occurred simultaneously as
compared to the isolated photocatalytic process only. The isolated photocatalytic process
of RR4, MB and phenol by PP0.6-450 seemed to be slow especially at the first 10
minutes of irradiation times but became faster beyond that irradiation times as can be
seen in Figures 4.9(a), (b) and (c) respectively. This is because the the pre-saturation of
the photocatalyst by the pollutants into the surface of PP0.6-450 particle makes the
function of the photocatalytic process became slower [105].
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Figure 4.8: Adsorption study for pristine TiO2-P25 and PP0.6-450 for various types
of pollutants for 1 hour of adsorption process.
a)
0
20
40
60
80
100
0 5 10 15 20 25 30 35
Times (min)
%
RR4Remaining..
PP0.6-450 (Photo + Ads)
PP0.6-450 (Photo)
Time (minutes)
0
20
40
60
80
100
RR4 MB Phenol
Adsorp
tio
P25
PP0.6-450
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b)
c)
Figure 4.9: Photocatalyticdegradation of the pollutants: (a) 30 mg L-1
RR4, (b) 12
mg L-1
MB and (c) 10 mg L-1
phenol using presaturated PP0.6-450 and
normal PP0.4-450.
0
20
40
60
80
100
0 10 20 30 40 50 60Times (min)
%MBremaining..
PP0.6-450 (photo)
PP0.6-450 (photo+Ads)
0
20
40
60
80
100
0 15 30 45 60 75 90 105
Times (min)
%
Phenolrem
aining..
PP0.6-450 (Photo)
PP0.6-450 (Photo+ Ads)
Time (minutes)
Time (minutes)
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This is true since the adsorped pollutant shielded the surface of the photocatalyst
from light irradiation. After 10 minutes of irradiation, the photocatalytic degradation
become faster due to the partial removal of the adsorbed pollutant in the surface of
PP0.6-450 and thus allowed better exposure of the catalyst surface to the incoming light
irradiation.
4.3.2 Photodegradation of RR4, MB and phenol under different light sources.
The 45 W fluorescent lamp has a UV and visible light irradiance of about 3.5 and
311Wm-2
respectively. The UV irradiance becomes zero (0.0 W m-2
) when the UV filter
was attached to the 45 W fluorescent lamp and under this condition, the visible light
reading was 265 W m-2
. A 125 W fluorescent lamp attached with UV filter was also
used as a source of visible light at higher irradiation intensity where the visible light
irradiance was measured to be 360 W m-2
with no detected UV leakage.
Figures 4.10(a), (b), and (c) show the photocatalytic degradation of RR4, MB
and phenol under different light sources of irradiation by PP0.6-450 and pristine TiO2-
P25. It was found that PP0.6-450 has a good potocatalytic activity as compared with
pristine P25. Sample PP0.6-450 took as fast as 30, 90 and 120 minutes to complete the
degradation of RR4, MB and phenol respectively while pristine TiO2-P25 took longer
irradiation time to complete the degradation of those pollutants. The pseudo first order
rate constant (k) values for the degradation of RR4 by PP0.6-450 was almost 2 times
faster than the pristine TiO2-P25 which was 0.146 and 0.077 min-1 respectively. The
rate of degradation for MB and phenol increased by as much as 1.5 and 1.3 times for
PP0.6-450 when compared against pristine TiO2-P25. As expected, high photocatalytic
activity of PP0.6-450 was due to the presence of carbon as the electron scavenger that
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reduced the electron-hole recombination during photocatalytic process as confirmed by
the lowest PL intensity of PP0.6-450. Same observation was reported by Janus et al. [74]
where the carbon coated on TiO2 can photoaccelerate the process by acting as an
electron scavenger.
The photocatalytic degradation of RR4 under visible light irradiation is displayed
in Figures 4.10(a), (b) and (c). In Figure 4.10(a), both PP0.6-450 and pristine TiO2-
P25 samples degraded RR4 dye by about 40 and 60% respectively but no further
degradation was observed beyond that level even after prolong irradiation. Apparently
it can be assumed that the removal of RR4 dye here was from the adsorption process
which correlated to the data observed in Figure 4.8 where only adsorption process has
occurred. The same trend (no photocatalytic degradation) was observed when PP0.6-450
and pristine TiO2-P25 was applied for the removal of MB and phenol under visible light
irradiation as can be seen in Figures 4.10(b) and (c).
When a 125-W fluorescent lamp attached with UV filter was used instead to study
the degradation of RR4, MB and phenol (Figures 4.10a, b and c), photocatalytic activity
by PP0.6-450 was observed for RR4 where 79% was removed after 30 minutes of
irradiation. In Figures 4.10(b) and (c), the photocatalytic activity of PP0.6-450 was also
observed for MB and phenol under similar high intensity visible light irradiation where
higher percentages of MB and phenol removal occurred. For pristine P25, no
photocatalytic activity was found for RR4, MB and phenol even under high intensity
visible light irradiation.
It can be inferred from the previous results that PP0.6-450 sample is a visible light
active photocatalyst that could degrade RR4, MB and phenol under visible light
irradiation. However the photocatalytic activity was not strong enough for it to function
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under low intensity visible light source (a 45 W fluorescent lamp) with visible
irradiation of only 260W m-2
. The visible light activity of PP0.6-450 sample was
actually predicted by its UV-Vis diffuse reflectance spectrum as shown in Figure 4.3.
PP0.6-450 sample has absorption within visible light range where its calculated bandgap
energy was about 2.8 eV (Figure 4.3b).
The effect of nitrogen doping into P25 is mainly the reason of its lower bangap
energy and similar observation had been documented in the literatures [47, 119]. As can
be seen in Figures 4.10 (a), (b) and (C), both samples (pristine P25 and PP0.6-450) had
excellence rate of photodegradation of RR4, MB and phenol under solar irradiation
where PP0.6-450 sample was always faster than pristine TiO2-P25 in the removal of
those pollutants.
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a.)
0
20
40
60
80
100
0 5 10 15 20 25 30
%
RR4remaining..
P25- Lamp
PP0.6-450- Lamp
P25- Lamp-UV Filter
PP0.6-450-UV Filter
P25-125W- UV Filter
PP0.6-450-125W-UV Filter
P25- Solar
PP0.6-450- Solar
Time (minutes)
P25 (45 W)
PP0.6-450 (45 W)
P25 45 W (Visible Light)
PP0.6-450 45 W (Visible Light)
P25, 125 W (V. Light)
PP0.6-450 125 W (Visible Light)
P25 (Solar)
PP0.6-450 (Solar)
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b.)
0
20
40
60
80
100
0 15 30 45 60
%MBremaining..
PP0.6-450- UV filter
P25- UV filter
PP0.6-450- 45W Lamp
P25- 45W Lamp
PP0.6-450- Solar
P25- Solar
PP0.6-450- 125W LampP25- 125W Lamp
Time (minutes)
PP0.6-450 45 W (V. Light)
P25 45 W (V. Light)
PP0.6-450 45 W
P25 45 W
PP0.6-450 (Solar)
P25 (Solar)
PP0.6-450 125 W (V. Light)
P25 125 W (V. Light)
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c.)
Figure 4.10:Photocatalytic degradation of various type pollutans: (a) RR4, (b) MB and(c) phenol under pristine TiO2-P25 and PP0.6-450 at different types of
irradiation.
0
20
40
60
80
100
0 15 30 45 60 75 90 105 120
%Phenolremaining..
PP0.6-450- UV Filter
P25- UV Filter
PP0.6-450- Lamp
P25- Lamp
PP0.6-450- Solar
P25- Solar
PP0.6-450-125W-UV FilterP25-125W- UV Filter
Time (minutes)
PP0.6-450 45 W (Visible Light)
P25 45 W (Visible Light)
PP0.6-450 45 W
P25 45 W
PP0.6-450 (Solar)
P25 (Solar)
PP0.6-450 125 W (Visible Light)
P25 125 W (Visible Light)
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4.4 The Operational Parameters Governing the Photocatalytic Degradation of
RR4 Dye by the C coated TiO2-P25
4.4.1 The effect of initial dye concentration
Figure 4.11a shows the effect of photocatalytic degradation of RR4 dye at various
initial dye concentrations using PP0.6-450 sample. The photocatalytic activity of RR4
dye decreased with increasing dye concentration. 5ppm RR4 dye solution was the fastest
to be degraded where the pseudo first order rate constant was 0.291 min-1
. Beyond this
concentration, the rate becomes slower with values at. 0.235, 0.160 and 0.085 min-1
for
10, 30 and 60 mg L-1 respectively (Figure 4.11b). The photocatalytic activity of PP0.6-
450 decreased with increasing initial concentration of the dye due to the increasing
initial color intensity of the solution (Figure 4.11c).
A significant amount of photons from the light irradiation may be absorbed by the
dye molecules rather than the TiO2 particles thus producing the light-screening effect
that retarded the penetration of light into the surface of the photocatalyst. As a result,
the formation of OH
(hydroxyl) and O2-(superoxide) radicals was reduced which
eventually decreased the efficiency of the photocatalyst.
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a)
0
20
40
60
80
100
0 5 10 15 20 25
Times (min)
%
RR4Remainin
5 mg/L
10 mg/L
30 mg/L
60 mg/L
Time (minutes)
%R
R4Remaining
10 m L-
30 m L-
60 m L-
5 m L-
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149
b)
c)
Figure 4.11: Effect of different concentration in terms of: (a) photodegradation of
RR4, (b) pseudo first order rate constant of RR4 using PP0.6-450 sample
and (c) the observation of the color intense for RR4 at differentconcentration.
10 mg L- 30 mg L- 60 mg L-15 mg L-1
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
5mg/L 10mg/L 30mg/L 60mg/L
1storderrateconst.K/min..
10 mg L-
5 mg L-
30 mg L-
60 mg L-
1storderratec
onst.(min-1)
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4.4.2 The effect of catalyst loading
It is important to identify the optimum amount of catalyst for the photocatalytic
reaction in order to avoid unnecessary used of excessive catalyst and also to ensure
optimum absorption of light photons for efficient photocatalytic degradation. Figure
4.12 shows the rate of 30 mg L-1
RR4 degradation under various amount of C coated N
doped TiO2-P25 (PP0.6-450) loading. It was observed that the degradation rate increased
with the increasing catalyst loading until 0.024 g. Beyond this point, no further
increment of RR4 removal was observed where the rate of RR4 removal became a
plateau. In line with the explanation from the Section 3.4.2, the increased TiO2 particles
in the solution is the main factor for increasing the rate of RR4 removal from 0.012 to
0.024g catalyst loading. By increasing the PP0.6-450 particles, the number of photon
and the dye molecules absorbed on the surface of the active site would also increase.
However, the reaction rate become stable as the amount of photocatalyst was increased
further beyond 0.024g.
There are two possibilities that influence this phenomenon: 1) This might be due
to the scattering of light and reduction in light penetration through the solution as a
result of the excess catalyst particles, 2) the aggregation of TiO2 particles at high
concentration caused a decrease in the number of surface active sites, thus bringing little
stimulation to the catalytic reaction [116].
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Figure 4.12: Pseudo first order rate constant of the degradation 20 mL of RR4 dye at
different loading of PP0.6-450 sample.
0
0.04
0.08
0.12
0.16
0.2
0 0.02 0.04 0.06 0.08
Catalyst loading (g)
1storderrateconst.,k/min
1storderrateconst.(min-1)
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4.4.3 The effect of aeration flow rate
All photocatalytic evaluations in this study were carried out under continuous
aeration. This was performed by using aquarium air pump as the source of aeration to
promote agitation of the aqueous solution as well as acting as the electron scanvenger
for the eventual production of superoxides radical anions. A series of experiments has
been carried out in order to study the role of oxygen and the effect of aeration flow rate
on the photocatalytic activity of the photocatalyst.
As shown in Figure 4.13, the optimum aeration rate for a C coated TiO2-P25 was
obtained at 25 mL min-1 where the pseudo first order rate constant (k) for the
degradation of RR4 was 0.158 min-1
compared to 0.127 min-1
for the degradation rate of
RR4 without aeration. By beyond the optimum aeration, the pseudo first order rate
constant was slightly comparable with optimum aeration due to the scattering effect
made by production of bubbles at higher aeration rate. All photocatalytic and adsorption
experiments in this work were therefore carried out under this aeration rate.
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153
Figure 4.13: Effect of aeration flow rate on the photocatalytic degradation of 30 mg L-
1RR4 by PP0.6-450 under a 45 W fluorescent lamp.
0.025
0.045
0.065
0.085
0.105
0.125
0.145
0.165
0 25 50 75 100
Aeration flow rate (mL/min)
1storderratecont.,k/mi
Aeration flow rate (mL min-1
)
1storderrateconst.
(min-1)
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4.4.4 The effect of initial pH
Figure 4.14 shows the rate of photodegradation of RR4 by C coated TiO 2-P25
(PP0.6-450) at different initial pH conditions as a function of the irradiation time. The
rate of photocatalytic degradation of RR4 at pH 2, 4 and 7 were slightly similar to each
other while the photocatalytic degradation of RR4 under pH 10 appeared to be very poor
where less than 50% of RR4 degraded after 15 minutes contact time. However,
degradation of RR4 was almost complete at pH 2, 4 and 7 under the same irradiation
times.
In line with our explanation in Section 3.4.4, photocatalytic activity becomes less
effective at basic condition (pH 10) due to the repulsion between the dye molecules and
PP0.6-450 particles which lead to decreased in the dye adsorption efficiency by the
ctatalyst particles. On the other hand, the photocatalytic degradation of RR4 becomes
faster at pH 2, 4 and 7 due to the increasing positive charges density on the surface of
the catalyst (refer the PZC value for about 4.6 in Figure 4.15) thus generated better
coulombic attraction with the negatively charged RR4 dyes. In addition, the increased
positive charge density also generates less agglomeration of PP0.6-450 particles which
also increased the adsorption process.
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Figure 4.15: Point of zero charge for C coated TiO2-P25, PP0.6-450.
-2.5
-2
-1.5
-1
-0.5
0
0.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5
PZC: 4.65
Initial pH
DiscrepancypH
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4.5 Mineralization Study of RR4, MB and Phenol by C coated TiO2-P25
Since TOC is much reliable than COD for mineralization study, in Sections 4 and
5 onward, TOC values will be discussed for mineralization study. Nevertheless, the
COD value was also accepted among the best way for mineralization study. TOC is
considered the most relevant or true parameter for the global determination of organic
pollution [160]. The TOC values indicate the presence of carbonaceous substances in
water samples. Thus the higher the value means the higher would be the presence of
carbonaceous substances. The lower the TOC values, the less would be the
carbonaceous material remaining in the solution. This means that the organic content of
the samples have been oxidized or mineralized into CO2 and H2O which do not
contribute to the TOC values. The TOC values of RR4, MB and phenol after
photocatalytic treatment with both pristine TiO2-P25 and PP0.6-450 samples using 45
Watt fluorescent lamp are given in Figure 4.16.
For all cases, the degrees of mineralization achieved by PP0.6-450 are much higher
than that of the pristine P25. For RR4, a complete mineralization was achieved by
PP0.6-450 after 6 hours of irradiation. However, for pristine TiO2-P25, about 50% of the
dye or 4 ppm of TOC remained at the same irradiation time. For MB, a complete
mineralization was achieved by PP0.6-450 while more than 60% of TOC still remained
for the pristine TiO2-P25 at 7 hours of irradiation. For phenol, a complete mineralization
was achieved with PP0.6-450 after 9 hours of irradiation, while 20 % of phenol still
remained for pristine TiO2-P25 under similar conditions.
It was observed that the rate of the mineralization of RR4, MB and phenol was
much slower as compared to the photocatalytic degradation of RR4, MB and phenol. In
line with our discussion in Section 3.5, based on the reported mechanism of MB and
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phenol degradation [142,143], several intermediates were initially formed from the
degradation of MB and phenol. For case of RR4 dye, no detailed reaction mechanism
was reported yet by other researchers. This can be strong evidence that mineralization
was much slower than photocatalytic degradation. Figure 4.17 shows several peaks of
intermediates detected from HPLC chromatograms of RR4 and Phenol during their
respective photodegradation by using PP0.6-450 sample.
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Figure 4.16:TOC values for photodegradation of RR4, MB and phenol using pristine
TiO2-P25 and PP0.6-450 irradiated with 45 Watt fluorescent lamp.
0
1
2
3
4
5
6
7
8
0 1 2 3 4 5 6 7 8 9 10
Times (h)
TOC,mg/L
Phenol ( p25)
Phenol (PC0.6-450)
RR4 (P25)
RR4 (PC0.6-450)
MB (PC0.6-450)
MB (P25)
TOC(mgL-1)
Time (h)
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a)
b)
Figure 4.17:HPLC chromatogram for a) RR4, b) phenol with intermediates
respectively.
Intermediates
3.478
3.837
6.239
7.154
8.816
Maleic Acid
Fumaric Acid
Hydroquinone
Catechol
Phenol
Intermediates
4.461
5.195
5.522
6.341
7.060
7.6921
8.175
4.027
Intermediates
RR4
Intermediates
Phenol
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4.6 The Stability of the PhotocatalystTable 4.3 shows the stability effect of carbon and nitrogen in TiO2-P25 against
photocatalytic degradation. The results were obtained when the suspended of C coated N
doped TiO2-P25 (PP0.6-450) particles in ultrapure water were exposed to prolonged
irradiations by the 45-W fluorescent lamp for 24, 48 and 72 hours respectively. After 72
hours of irradiation, the results show that the percentages C and N contents were clearly
maintained (Table 4.3).
To illustrate further the stability of C coated TiO2-P25, repeated reuse or recycling
of the photocatalyst in the degradation of RR4 dye was studied. The results are provided
in Figure 4.18 where the efficiency of RR4 removal by C coated TiO 2-P25 under
repeated reuse was found to be maintained at around 99.1% within 30 minutes of
irradiation (Figure 4.18). This evidence proves that a quite stable state was achieved for
carbon and nitrogen in TiO2-P25 particle.
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Table 4.3: Carbon content of C coated TiO2-P25 (PP0.6-450) upon prolonged
irradiations with a 45 W fluorescent lamp in distilled water for 0, 24, 48 and72 h.
Figure 4.18: Photocatalytic efficiency of PP0.6-450, C coated TiO2-P25 upon recycledapplications in the degradation of RR4.
Irradiation Times (Hours) 0 24 48 72
Carbon content
(%)
0.120 0.124 0.122 0.121
Nitrogen content
(%)
0.090 0.087 0.085 0.085
0
20
40
60
80
100
1 2 3 4 5
Number of cycles
%
Deg
radatio