chapter 7 photocatalytic activity of io -doped...

183

CHAPTER 7

PHOTOCATALYTIC ACTIVITY OF

IO3−−−−-DOPED TiO2

7.1 PHOTOCATALYTIC ACTIVITY OF IO3--DOPED TiO2 FOR

THE DEGRADATION OF MONOCROTOPHOS AND

2,4,6-TRICHLOROPHENOL IN AQUEOUS SUSPENSION

A great deal of effort has been devoted in recent years to develop

high activity heterogeneous photocatalysts for environmental applications

such as air purification, water disinfection, hazardous waste remediation and

wastewater treatment. Among the various semiconductor photocatalysts,

titania has proven to be the most suitable for widespread environmental

applications due to its biological and chemical inertness, strong oxidizing

power, cost effectiveness and long-term stability against photocorrosion and

chemical corrosion (Hoffmann et al 1995). The photocatalytic activity of

semiconductors is due to the production of excited electrons in the conduction

band of semiconductor along with corresponding positive holes in the valence

band by the absorption of UV illumination. These energetically excited

species are mobile and capable of initiating many chemical reactions, usually

by the production of radical species at the semiconductor surface. They are

unstable and recombination of photogenerated electron-hole can occur very

quickly, dissipating the input energy as heat.

In fact, the photocatalytic efficiency depends on the competition

between two processes, that is, the ratio of surface charge carrier transfer

184

rate to the electron-hole recombination rate. If recombination occurs fast

(< 0.1 ns), then there is not enough time for any other chemical reactions to

occur (Hoffmann et al 1995). In titania, the species are relatively long-lived

(around 250 ns), allowing the electron or hole to travel to the crystallite

surface. It is on the TiO2 surface that different types of radicals are formed.

The most common is the ●OH radical, which is then free to carry out further

chemical reaction at the titania surface (Ovenstone 2001). To reduce

recombination of photogenerated electron and holes and to extend its light

absorption into the visible region, various transition metal cations have been

doped into titania (Soria et al 1991; Choi et al 1994; Moon et al 2001). Choi

et al (1994) conducted a systematic study of metal ion doping in quantum

sized TiO2. They found that doping with Fe3+

, Mo5+

, Ru3+

, Os3+

, Re5+

, V4+

and

Rh3+

at 0.1 - 0.5 wt% significantly increased the photoactivity.

Similarly Wang and Mallouk (1990) reported the photocatalytic

fluorination of organic molecules on the surface of titanium dioxide by

adsorption of fluoride and hydrofluoric acid. Hattori et al (1998) reported

significant enhancement in the photocatalytic activity of TiO2 powder or thin

films by doping with F- ions. Luo et al (2004) reported enhancement in the

photocatalytic activity for titanium dioxide by co-doping with bromine and

chlorine. However, the effect of IO3–-doping in TiO2 for the photocatalytic

degradation of organic pollutants has not been reported so far. In the present

study, IO3--doped TiO2 catalysts were prepared and characterized by XRD,

SEM-EDX and UV-Vis analysis. The photocatalytic activity of the materials

was tested for the degradation of MCP and TCP, model pollutants and

endocrine disrupting chemicals.

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7.1.1 Characterisation of Photocatalysts

7.1.1.1 Effect of iodic acid in the crystallization of TiO2

The effect of iodic acid in the crystallization of TiO2 phase and

particle size was studied by XRD analysis (Figures 7.1 to 7.6). The results are

presented in Table 7.1. Crystallization of TiO2 yields mainly anatase and

rutile in the absence of iodic acid. But in the presence of iodic acid,

crystallization of brookite phase is also observed. The content of brookite

phase varies with the amount of iodic acid used during crystallisation. The

formation of brookite phase increases with an increase in iodic acid content

whereas the anatase and rutile phase content decreases with an increase in

iodic acid content. The average particle size of TiO2 in the absence of iodic

acid is higher than in the presence of iodic acid. But the size of TiO2 appears

to be nearly same in the presence of iodic acid. Thus the presence of iodic

acid during crystallization of TiO2 appears to influence significantly upon

phase composition and particle size.

Table 7.1 Physico-chemical properties of TiO2 and IO3--doped TiO2

Catalyst % of TiO2 phase Particle size

(nm) Anatase Rutile Brookite

TiO2 77.36 22.64 - 30.00

0.5wt% IO3--TiO2 77.9 22.1 7.52 25.20

0.7wt% IO3--TiO2 77.53 22.47 12.56 22.68

1.0wt% IO3--TiO2 63.32 18.15 18.53 22.42

1.5wt% IO3--TiO2 63.42 16.91 19.67 23.59

2.0wt% IO3--TiO2 64.27 17.16 18.56 24.01

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Figure 7.1 XRD pattern of TiO2

Figure 7.2 XRD pattern of 0.5 wt% IO3--doped TiO2

187



188



189

7.1.1.2 Ultraviolet-Visible (UV-Vis) absorption spectra of TiO2 and

IO3--doped TiO2

The UV-Vis spectra of TiO2 and different loadings of IO3−-doped

TiO2 were recorded in the range 200-900 nm. The spectra are depicted in

Figure 7.7. TiO2 shows broad absorbance band below 400 nm. The shift of

onset of absorbance towards longer wavelength for TiO2 crystallised in the

presence of iodic acid is clearly evident in comparison to parent TiO2.

Although the effect does not appear to be linear, the difference is clearly

evident. The shift may not be attributed to any isomorphic substitution of

oxidic sites of TiO2 by iodic acid as the ionic radius of later is higher than

TiO2. Conversely iodic acid may be present as interstitial impurity in the

lattice of TiO2. The small difference in particle size as shown in Table 7.1

may also be a contributing factor for the shift of absorbance to longer

wavelength.

Figure 7.7 UV-Vis absorption spectra of TiO2 and IO3--doped TiO2

190

a

7.1.1.3 SEM-EDX analysis of TiO2 and IO3--doped TiO2

The SEM-EDX spectra of TiO2 and 1 wt% IO3--doped TiO2 are

shown in Figures 7.8a and 7.8b respectively. The SEM-EDX results for TiO2

shows only the presence of Ti and O in it whereas 1 wt% IO3--doped TiO2

clearly shows the presence of iodine, Ti and O. Hence IO3- as lattice

constituent of TiO2 is clearly evident from this analysis. The analytical results

from EDX are in reasonable agreement with the nominal 1 wt% IO3--doped

into TiO2.

Figure 7.8 SEM-EDX spectra of (a) TiO2 and (b) 1 wt% IO3−−−−-doped

TiO2

b

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7.1.2 Photocatalytic Activity of TiO2 and IO3--Doped TiO2 for the

Degradation of MCP and TCP in Aqueous Suspension

The photocatalytic activity of TiO2 and IO3--doped TiO2 was

studied for the degradation of MCP with the light of wavelengths 254 and

365 nm and the results are presented in Tables 7.2 and 7.3. The degradation of

MCP follows psuedo-first order kinetics. The activity of parent TiO2 is low

for light of wavelength 254 nm. IO3−-doped (1 wt%) TiO2 shows higher rate

constant than other wt% IO3--doped TiO2. The reason could be explained on

the basis of formation of brookite phase. The brookite phase increases with

increase in iodic acid loading with simultaneous decrease in the percentage of

anatase and rutile phases. It was observed that 18.5% brookite phase is

present in 1 wt% IO3--doped TiO2. This is in good agreement with the concept

of mixed phases enhances the photocatalytic activity as reported by Luo et al

(2004). This study also concludes that the presence of all three phases of TiO2

with small particle size under optimum loading of iodic acid may be

important for the enhanced photocatalytic activity. The t1/2 values of TiO2 and

IO3--doped TiO2 (Table 7.2) also illustrate that 1 wt% IO3

--doped TiO2 is more

active catalyst than other catalysts. The t1/2 value of this catalyst is lower than

all other catalysts. The rate constants and t1/2 values with light of wavelength

365 nm for the degradation of MCP are shown in Table 7.3. IO3--doped TiO2

catalysts are more active than TiO2. The t1/2 values of 1 wt% IO3--doped TiO2

is lower than all other catalysts. This study also revealed the optimum loading

of iodic acid is 1 wt%.

The photocatalytic degradation of TCP over TiO2 and IO3−-doped

TiO2 catalysts was also studied with light of wavelengths 254 and

365 nm and the results are presented in Tables 7.4 and 7.5. The degradation of

TCP also shows higher activity for IO3−-doped TiO2 than TiO2 as that of MCP.

Based on the rate constants and t1/2 values, the optimum loading is

found to be 1 wt%. The degradation of TCP also shows less rate constants for

light of wavelength 365 nm.

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Table 7.2 Apparent reaction rate constants and t½ values for the

degradation of MCP with 254 nm

Catalyst

Apparent reaction

rate constant

k(x 10-2

min-1

)

t1/2 values

(min)

Correlation

co-efficient

(R2 value)

TiO2 8.50 8.15 0.9952

0. wt% IO3--TiO2 16.97 4.08 0.9854

0.7 wt% IO3--TiO2 21.07 3.29 0.9984

1.0 wt% IO3--TiO2 21.40 3.23 0.9854

1.5 wt% IO3--TiO2 20.69 3.35 0.9848

2.0 wt% IO3--TiO2 17.97 3.85 0.9559

MCP = 40 mg l-1

, TiO2 or IO3--TiO2 = 100 mg/100 ml, pH = 5, UV = 8 lamps,

λ = 254 nm, Adsorption equilibrium time = 30 min and Irradiation

time = 60 min


degradation of MCP with 365 nm

Catalyst

Apparent reaction

rate constant

k (x 10-2

min-1

)

t1/2 values

(min)

Correlation

co-efficient

(R2 value)

TiO2 7.5 9.24 0.9976

0.5wt% IO3--TiO2 13.37 5.18 0.9800

0.7wt% IO3--TiO2 13.89 4.98 0.9978

1.0wt% IO3--TiO2 15.25 4.54 0.9777

1.5wt% IO3--TiO2 14.7 4.71 0.9833

2.0wt% IO3--TiO2 13.0 5.33 0.9938

MCP = 40 mg l-1



time = 60 min

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degradation of TCP with 254 nm

Catalyst

Apparent reaction

rate constant

k(x 10-2

min-1

)

t1/2 values

(min)

Correlation

co-efficient

(R2 value)

TiO2 11.00 6.3 0.991

0.5wt% IO3--TiO2 21.01 3.3 0.994

0.7wt% IO3--TiO2 25.03 2.8 0.987

1.0wt% IO3--TiO2 31.00 2.2 0.995

1.5wt% IO3--TiO2 26.05 2.7 0.993

2.0wt% IO3--TiO2 23.01 3.0 0.992

TCP = 40 mg l-1



time = 60 min


degradation of TCP with 365 nm

Catalyst

Apparent reaction

rate constant

k (x 10-2

min-1

)

t1/2 values

(min)

Correlation

co-efficient

(R2 value)

TiO2 9.02 7.7 0.996

0.5wt% IO3--TiO2 15.00 4.6 0.982

0.7wt% IO3--TiO2 17.12 4.1 0.997

1.0wt% IO3--TiO2 21.00 3.3 0.993

1.5wt% IO3--TiO2 19.06 3.6 0.989

2.0 wt% IO3--TiO2 16.08 4.3 0.990

TCP = 40 mg l-1



time = 60 min

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7.1.3 Photocatalytic Mineralization of MCP and TCP in Aqueous

Suspension

The effect of irradiation time on TOC was studied for the

degradation of MCP and TCP with the lights of wavelengths 254 and 365 nm

and the results are illustrated in Figures 7.9 to 7.12. There is rapid decrease in

TOC for 1 wt% IO3--doped TiO2 whereas the decrease is very slow for TiO2.

With increasing iodic acid content in TiO2, the rate of decrease of TOC

increases steadily upto 1.0 wt% and above 1 wt% the rate of decrease is slow.

Similar observations are also observed for 365 nm (Figure 7.10). The rate of

decrease of TOC in respect of MCP and TCP was less for 365 nm than

254 nm. The degradation of MCP and TCP with TiO2 is very less for both

light of wavelengths 254 and 365 nm.

Figure 7.9 Comparison of photocatalytic mineralisation with TiO2 and

IO3--doped TiO2 (MCP concentration = 40 mg l

-1, catalyst

amount = 100 mg/100 ml, solution pH = 5 and λ = 254 nm)

195

Figure 7.10 Comparison of photocatalytic mineralisation with TiO2 and

IO3--doped TiO2 (MCP concentration = 40 mg l

-1, catalyst


Figure 7.11 Comparison of photocatalytic mineralization with TiO2 and

IO3--doped TiO2 (TCP concentration = 40 mg l

-1, catalyst


196

Figure 7.12 Comparison of photocatalytic mineralization with TiO2 and

IO3--doped TiO2 (TCP concentration = 40 mg l

-1, catalyst


7.1.4 Relative Photonic Efficiency

Relative photonic efficiencies of TiO2 and IO3--doped TiO2 for the

degradation of MCP was determined by mixing a solution of 40 mg l-1

MCP

or TCP adjusted to pH 5 and 100 mg of TiO2 or IO3--doped TiO2 and

irradiated for 60 min. The relative photonic efficiencies of TiO2 and

IO3--doped TiO2 for the degradation of MCP and TCP are shown in Tables

7.6 and 7.7 respectively. The data clearly indicate that 1 wt% IO3--doped TiO2

is more active catalyst than other catalysts. The relative photonic efficiency is

about 2.5 times higher than that of Degussa P-25 for the degradation of MCP.

1 wt% IO3−-doped TiO2 shows 2.99 times higher relative photonic efficiency

than Degussa P-25 in the degradation of TCP.

197

Table 7.6 Comparison of relative photonic efficiencies in the

photodegradation of MCP by TiO2, IO3--doped TiO2 and

commercial photocatalysts

Sl.

No.

Catalyst Relative photonic efficiency

(ξξξξr)

1. Degussa P-25 1.00 ± 0.03

2. 1wt%IO3--TiO2(254 nm) 2.55 ± 0.01

3. 1wt%IO3--TiO2(365 nm) 2.0 ± 0.01

4. Degussa P-25*

1.0 ± 0.1

*5. Baker & Adamson 0.38 ± 0.02

6 Ti-oxide 1.90 ± 0.1

7. Sargent-Weich 2.1 ± 0.1

8. Fluka AG 2.2 ± 0.1

9. Hombikat UV-100 0.25 ± 0.02

* (Sl.No. 4-9) Serpone, J. Photochem. Photobiol. A: Chem., 104 (1997) 1-12

MCP = 40 mg l-1

, TiO2 or IO3−-TiO2 = 100 mg/100 ml, pH = 5, UV = 8 lamps,

λ = 254 or 365 nm, Adsorption equilibrium time = 30 min and Irradiation

time = 60 min

198

Table 7.7 Comparison of relative photonic efficiencies in the

photodegradation of TCP by TiO2, IO3--doped TiO2 and

commercial photocatalysts

Sl.

No.

Catalyst Relative photonic efficiency

(ξξξξr)

1. Degussa P-25 1.00 ± 0.03

2. 1wt% IO3--TiO2(254 nm) 2.99 ± 0.01

3. 1wt% IO3--TiO2(365 nm) 2.43 ± 0.01

4. Degussa P-25*

1.00 ± 0.1

*5. Baker & Adamson 0.38 ± 0.02

6 Ti-oxide 1.9± 0.1

7. Sargent-Weich 2.1 ± 0.1

8. Fluka AG 2.2 ± 0.1

9. Hombikat UV-100 0.25 ± 0.02

* (Sl.No. 4-9) Serpone, J. Photochem. Photobiol. A: Chem., 104 (1997) 1-12

TCP = 40 mg l-1

, TiO2 or IO3−-TiO2 = 100 mg/100 ml, pH = 5, UV = 8 lamps,

λ = 254 or 365 nm, Adsorption equilibrium time = 30 min and Irradiation

time = 60 min

7.2 COMPARISON OF DEGRADATION OF MCP AND TCP

The rate constant for the degradation of MCP with 1 wt%

IO3--doped TiO2 with light of wavelength 254 nm is 0.143 mg l

-1sec

-1 whereas

with 365 nm it is only 0.101 mg l-1

sec-1

. The rate constant for the degradation

of TCP with 1 wt% IO3--doped TiO2 is 0.21 mg l

-1sec

-1 with light of

wavelength 254 nm but the rate constant with light of wavelength 365 nm is

0.14 mg l-1

sec-1

. The relative photonic efficiency for the degradation of MCP

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with light of wavelength 254 nm is 2.5 ± 0.01 whereas the same for the

degradation of TCP with light of wavelength of 254 nm is 2.99 ± 0.01. The

above results revealed that TCP is more prone to photocatalytic degradation

than MCP under identical experimental conditions. The small molecular size

of TCP can aid better adsorption on the catalyst surface thus giving high rate

of degradation.

This study concludes that IO3−-doped TiO2 catalysts are more

active than TiO2. Although iodic acid in TiO2 lattice is not clearly evident

from the XRD analysis, its presence is clearly evident from the

SEM-EDX analysis. The rate constants for the degradation of MCP and TCP

with iodic acid also confirm the existence of iodic acid in the lattice of TiO2.

The rate constants are higher for IO3−-doped TiO2 than TiO2. The decrease of

TOC with irradiation time for the degradation of MCP and TCP is quite

significant for IO3--doped TiO2. This supports the presence of IO3

- in the

lattice of TiO2. The relative photonic efficiencies of IO3--doped TiO2 are

higher than Degussa P-25. The formation of mixed phase (anatase, rutile and

brookite) and entry of iodic acid into the lattice of TiO2 are suggested to be

the cause for high activity of IO3−-doped TiO2.

chapter 7 photocatalytic activity of io -doped...

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