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OH
O
2
-
Photo-reduction
Energy
O
2
UV
-
OH + OH
e
-
+ H
2
O
2
e
-
CB
TiO
2
H
2
O/
-
OH; R
h
+
VB
Photo-oxidation
OH; R +
OH + R
intermediates
CO
2
+
H
2
O
FIGURE 6.9
Schematic diagram illustrating the principle of TiO
2
photocatalysis. (Reprinted from
J. Environ. Manag.,
92,
Ahmed,S.,Rasul,M.G.,Brown,R.,andHashib,M.A.,Inluenceofparametersontheheterogeneousphotocata-
lyticdegradationofpesticidesandphenoliccontaminantsinwastewater:Ashortreview,311-330,(2011),with
permissionfromElsevier.)
(6.10)
e O
−
+ →
•
O
−
2
2
Inthisreaction,h
+
ande
−
arepowerfuloxidizingandreducingagents,respectively.The
oxidativeandreductivereactionstepsareexpressedas(Ahmedetal.2011)
Oxidativereaction
( )
→
h Organic R
+
+
Intermediates
→
CO H O
2
+
(6.11)
2
h H O
+
+
→
•
OH H
+
+
(6.12)
2
Reductivereaction
( )
→
•OH Organic R
+
Intermediates
→
CO H O
2
+
(6.13)
2
Hydroxyl radical generation by the photocatalytic oxidation process is shown in the
above steps. The resulting intermediates further react with •OH to produce inal deg-
radation products such as CO
2
and H
2
O. The details are described in the review paper
by Ahmed et al. (2011). The photocatalytic activity of TiO
2
depends on the surface and
structuralsemiconductorpropertiessuchascrystalcomposition,surfacearea,particlesize
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