Environmental Engineering Reference
In-Depth Information
portion of the substrate is oxidized by valance band holes or conduct band electrons
(versus trapped electrons and holes) due to the limited amount and short lifetime of the
excitons (Martin et al., 1994a). However, some literatures have reported that valence-
band hole reaction may occur at the surface before it is trapped or recombined (Richard,
1993).
According to this mechanism, the quantum efficiency for interfacial redox
reaction is determined by two critical processes. First, the competition between charge-
carrier recombination (Eqs. 3.5 and 3.6) and trapping (Eqs. 3.23.4) in pico-second to
nano-second time scale. Second, the competition between the trapped carrier
recombination (Eqs. 3.5 and 3.6) and interfacial charge transfer process (Eqs. 3.7 and
3.8), which is in micro-second to milli-second scale (Hoffmann et al., 1995). An
increase in either the lifetime of charge carriers or the interfacial charge transfer rate
constant is able to increase the overall quantum efficiency of the photolysis reaction. In
general, the presence of dopants hinders the recombination process, which in turn
prolongs the charge carriers' lifetime. However, as a side effect, a decrease of
interfacial charge transfer rate takes place in the presence of dopants. This has been
approved experimentally by Hoffmann and co-workers using the time-resolved
microwave conductivity measurements (Martin et al., 1994a). Therefore, the role of
dopants should be well balanced in order to maximize the quantum efficiency.
ROS and other radical species, including reducing radicals, can also be formed
and participate in the degradation reaction indirectly. Oxidation of adsorbed water
and/or hydroxyl groups by trapped electron holes can give rise to formation of highly
reactive ROS, such as OH· as shown below (Konstantinou and Albanis, 2004):
Formation of Oxidizing ROS
h
tr
+
+ H
2
O H
+
+ OH
•
(Eq. 3.9)
h
tr
+
+ OH
-
OH
•
(Eq. 3.10)
In the presence of proper electron donors, reducing ROS can be formed by a
conduction-band electron or trapped electron reduction through a series of reactions, as
illustrated below (Hoffmann et al., 1995; Demeestere et al., 2007):
Formation of Reducing ROS
e
tr
-
+ O
2
O
2
•
-
(Eq. 3.11)
O
2
•
-
+ H
3
O
+
HO
2
• +
H
2
O
(Eq. 3.12)
2HO
2
•
H
2
O
2
+ O
2
(Eq. 3.13)
e
tr
-
+ H
2
O
2
OH
•
+ OH
-
(Eq. 3.14)
However, in general, the photocatalytic reduction process is less important than
the oxidation reaction, as the reduction potential of e
tr
-
is lower than the oxidation
Search WWH ::
Custom Search