Civil Engineering Reference
In-Depth Information
increase in TiO
2
publications per year demonstrates the growth in the
area.
10,11
14.1.1 Photocatalysis
The term photocatalysis implies that light is acting as a catalyst in a reaction,
which is not the case.
12,13
However, the term photocatalysis will always be
used to describe the process that semiconductor materials such as TiO
2
undergo when irradiated by light of a certain wavelength. It is a term that
implies photon-assisted generation of catalytically active species.
In photocatalysis light of energy greater than the band gap of the semi-
conductor excites an electron from the valence band to the conduction band
(Fig. 14.1) by the following reaction:.
Semiconductor
+→
h
v
h
+
+
e
−
Scheme 14.1
VB
CB
The excited electron leaves a positive hole in the valence band and these
charge carriers can migrate to the catalyst surface and initiate redox reac-
tions on absorbents such as water and oxygen. Positive holes generated by
light become trapped by surface adsorbed H
2
O. The H
2
O becomes oxidized
by h
+
VB
, producing H
+
and OH• radicals (Eq. [14.1]), which are extremely
powerful oxidants (Table 14.1). The hydroxyl radicals subsequently oxidize
organic species from the surrounding environment to CO
2
and H
2
O (Eq.
[14.3])
14
and in most cases these are the most important radicals formed in
TiO
2
photocatalysis.
Electrons in the conduction band can be rapidly trapped by molecular
oxygen adsorbed on the particle. Trapped molecular oxygen will be reduced
by excited electrons to form superoxide (O
2−
•) radicals (Eq. [14.2]) that
Reduction:
O
2
+ e
-
O
2-
→
Conduction band
Super oxide
radical
-
electron
h
< 390 nm
ν
Band gap (E
g
)
Oxidation:
+
hole
H
2
O + h
+
OH
→
Hydroxyl radical
Valence band
14.1
Schematic of photocatalytic mechanism.
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