Civil Engineering Reference
In-Depth Information
Table14.1
Oxidation potentials of various oxidants
relative to normal hydrogen electrode (NHE)
Oxidant
Oxidation potential (V)
OH (hydroxyl radical)
2.80
O
3
(ozone)
2.07
H
2
O
2
(hydrogen peroxide)
1.77
HClO (hypochlorous acid)
1.49
Cl (chlorine)
1.36
may further react with H
+
(Eq. [14.4]), to generate peroxide radicals (•OOH)
and hydrogen peroxide, H
2
O
2
(Eq. [14.5]).
15,16
Recombination of the excited electron and the photo-generated hole is
a major limitation in semiconductor photocatalysis as it reduces the overall
quantum effi ciency of the photocatalyst because of the high recombination
rate of photoinduced electron-hole pairs at the surface of the photocata-
lyst.
17
The photocatalytic effi ciency can be signifi cantly enhanced if recom-
bination is reduced. Doping with ions,
18-20
heterojunction coupling
21
and
nanosized crystals
22
have all been reported to promote separation of the
electron-hole pair, reducing recombination and therefore improving the
photocatalytic activity of the semiconductor material.
When recombination occurs, the excited electron reverts to the valence
band without reacting with adsorbed species (Scheme 14.2).
23
Radiation
may be emitted when an excited electron recombines with the valence band.
As such, photoluminescence may be successfully employed to monitor
recombination and, in general, low intensity photoluminescence signals
indicate lower recombination rates.
17
e
−
+
h
+
→
energy
Scheme 14.2
CB
VB
HO h
+
+
→+
OH H
+
[14.1]
i
2
VB
Oe
+
−
→
O-
−
[14.2]
i
2
CB
2
OH
+
VOC
→+
2
H O
CO
2
[14.3]
i
O-
2
−
+→
H
+
-
OOH
[14.4]
i
i
OOH
+
OOH
→+
22
H O
O
2
[14.5]
i
i
O-
−
+
VOC
→+
CO
HO
[14.6]
i
2
2
2
OOH
+
VOC
→+
2
CO
H O
[14.7]
i
2
Equations [14.1]-[14.7] schematize the whole process.
14,24,25
Hydroxyl radi-
cals produced by the photocatalytic process will oxidize the majority
of volatile organic compounds (VOC) until complete mineralization.
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