Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
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.
185
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
186
Figure 7.1 XRD pattern of TiO2
Figure 7.2 XRD pattern of 0.5 wt% IO3--doped TiO2
187
Figure 7.3 XRD pattern of 0.7 wt% IO3--doped TiO2
Figure 7.4 XRD pattern of 1.0 wt% IO3--doped TiO2
188
Figure 7.5 XRD pattern of 1.5 wt% IO3--doped TiO2
Figure 7.6 XRD pattern of 2.0 wt% IO3--doped TiO2
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
191
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.
192
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
Table 7.3 Apparent reaction rate constants and t½ values for the
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
, TiO2 or IO3--TiO2 = 100 mg/100 ml, pH = 5, UV = 8 lamps,
λ = 365 nm, Adsorption equilibrium time = 30 min and Irradiation
time = 60 min
193
Table 7.4 Apparent reaction rate constants and t½ values for the
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
, 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
Table 7.5 Apparent reaction rate constants and t½ values for the
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
, TiO2 or IO3--TiO2 = 100 mg/100 ml, pH = 5, UV = 8 lamps,
λ = 365 nm, Adsorption equilibrium time = 30 min and Irradiation
time = 60 min
194
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
amount = 100 mg/100 ml, solution pH = 5 and λ = 365 nm)
Figure 7.11 Comparison of photocatalytic mineralization with TiO2 and
IO3--doped TiO2 (TCP concentration = 40 mg l
-1, catalyst
amount = 100 mg/100 ml, solution pH = 5 and λ = 254 nm)
196
Figure 7.12 Comparison of photocatalytic mineralization with TiO2 and
IO3--doped TiO2 (TCP concentration = 40 mg l
-1, catalyst
amount = 100 mg/100 ml, solution pH = 5 and λ = 365 nm)
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
199
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.