12) CHAPTER 4 (rehwtvised)-119-162

7/27/2019 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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141

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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143

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)


P25 (Solar)

PP0.6-450 (Solar)


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145

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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146

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

P25 45 W

PP0.6-450 (Solar)

P25 (Solar)




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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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148

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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150

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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151

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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152

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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154

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.


37/44


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156

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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157

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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158

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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159

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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160

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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161

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

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