Environmental Engineering Reference
In-Depth Information
Conduction band
e
-
E
c
E
g
Band gap
E
v
h
+
Valence band
FIGURE 6.24
Valence and conduction bands in a metal oxide semiconductor.
pair (
e
−
+
h
+
)
is depicted as
hn
−→
TiO
2
(e
−
+
h
+
)
.
TiO
2
Two types of oxidation reactions have been noted in aqueous suspensions of TiO
2
.
Oneistheelectrontransferfromtheorganicmoleculeadsorbedonthesurface(RX
ads
)
TiO
2
(h
+
)
RX
+•
+
RX
ads
−→
TiO
2
+
ads
,
and the second is the
e
−
transfer to the adsorbed solvent molecule (H
2
O
ads
)
TiO
2
(h
+
)
HO
ads
+
H
+
.
+
H
2
O
ads
−→
TiO
2
+
The above reaction appears to be of greater importance in the oxidation of organic
compounds. The abundance of water makes this a feasible reaction. Similarly,
adsorbed hydroxide ion (OH
ads
) also appears to participate in the reaction
TiO
2
(h
+
)
OH
ads
→
HO
ads
.
+
TiO
2
+
The ever-present O
2
in water provides another avenue of reaction by acting as an
electron acceptor:
TiO
2
(e
−
)
+
O
−•
2
O
2
→
TiO
2
+
.
The addition of hydrogen peroxide has been shown to catalyze this process, presum-
ably via a surface dissociation of H
2
O
2
to OH
•
that subsequently oxidizes organic
compounds.
TiO
2
(e
−
)
OH
−
+
OH
•
.
+
H
2
O
2
→
TiO
2
+
The above reaction is sensitive to pH changes and the electrical double-layer
properties of TiO
2
in aqueous solutions. Therefore, the UV-promoted dissociation
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