Environmental Engineering Reference
In-Depth Information
in the degradation reaction of the organic molecules or be further reduced to hydrogen
peroxide (H
2
O
2
). Owing to the electrophilic property, oxygen plays a decisive role in the
mechanism of the photocatalytic degradation. Photocatalytic eficiency can be enhanced
by the addition of H
2
O
2
. However, H
2
O
2
has a dual effect on the photocatalytic degradation
rate of organic substrates. Excess H
2
O
2
, which acts as an
∙
OH scavenger, produces an inhib-
itory effect in the photocatalytic eficiency. Electron scavenging and the consequent e
−
-h
+
recombination suppression can also be achieved by the use of other inorganic oxidants.
The use of an optimum concentration of inorganic peroxides has been demonstrated to
enhance the degradation rate of several organic pollutants.
25.6.7 Photocatalyst Deactivation
Deactivation is an important issue for practical applications of photocatalysts. Anions (Cl
−
,
ClO
2−
,
NO
3
−
,
CO
2−
,
HCO
3
−
,
SO
2−
,
PO
3−
) commonly found in neutral or polluted waters
retard the oxidation rate of organic compounds either by competing for radicals or by
blocking the active sites of the TiO
2
photocatalyst.
25.7 Modified TiO
2
for Improved Photocatalytic
Activity under Visible Light
Eficient utilization of visible and other solar light is one of the major goals of modern
science and engineering that will have a great impact on technological applications.
Although an AOPs with TiO
2
photocatalysts has been shown to be an effective alternative
in this regard, the vital snag of the TiO
2
semiconductor is that it absorbs a small portion
of the solar spectrum in the UV region (band-gap energy of TiO
2
is 3.2 eV). To utilize the
maximum solar energy, it is necessary to shift the absorption threshold toward the visible
region. Shifting of the TiO
2
absorption into the visible light region mainly focuses on the
doping with transition metals. Two general methods have been used to increase the photo-
catalytic activity of TiO
2
for visible light and solar light irradiation: (i) use of an organic dye
as photosensitizer and (ii) doping TiO
2
with metallic and nonmetallic elements.
25.7.1 Use of Organic Dye as Photosensitizer
Use of organic dye as a photosensitizer works very well under conditions where oxygen/air
is excluded and the degradation of the dye is minimized by the eficient quenching of the dye
oxidation state with an appropriate electrolyte. Otherwise, the dye becomes rapidly mineral-
ized and the photocatalytic system loses its response toward visible light in the presence of
oxygen. In dye sensitization, the most relevant points are (i) the absorption spectrum of the
dye in the visible region and (ii) the energy of the electron in the excited electronic state of the
dye, which has to be high enough to be transferred to the semiconductor conduction band.
25.7.2 Doping with Metal Ions
Transitional metal and rare earth metal ion doping have been extensively investigated for
enhancing TiO
2
photocatalytic activities. It was found that the doping of metal ions could
expand the photoresponse of TiO
2
into the visible spectrum.
