Environmental Engineering Reference
In-Depth Information
(PPCPs), pesticides, virus and bacteria (Mills and LeHunte, 1997; Hoffmann et al., 1995;
Kwon et al., 2008). In general, it is believed that two reactive pathways exist for
heterogeneous photocatalysis on TiO
2
, i.e. direct oxidation/reduction and indirect
oxidation by reactive oxygen species (ROS) or other radicals (Hoffmann et al., 1995;
Bhatkhande et al., 2002; Demeestere et al., 2007). Both reaction routes are supported by
a large body of literature with direct observation (Carraway et al., 1994; Martin et al.,
1994a, b) or indirect detection of intermediate products (Turchi and Ollis, 1990; Houas
et al., 2001).
Hoffmann and co-workers have proposed the classic direct reaction mechanism
and characteristic time scale of photocatalytic reactions based on time-resolved laser
flash photolysis measurements (Martin et al., 1994a), Table 3.2 lists the various
recombination reactions and characteristic times.
Table 3.2
Direct
photocatalytic redox processes and characteristic time (Martin et al.,
1994a) .
Primary Process
Characteristic Times
Charge-Carrier Generation
TiO
2
+
h
(> 3.2 eV) h
b
+
+ e
cb
-
10 ns (fast)
(Eq. 3.1)
Charge-Carrier Trapping
h
b
+
+ >Ti
IV
OH {>Ti
IV
OH•}
+
-h
tr
+
100 ps (shallow trap,
dynamic equilibrium)
(Eq. 3.2)
e
cb
-
+ >Ti
IV
OH {>Ti
III
OH}-e
tr
-
(Eq. 3.3)
e
cb
-
+ >Ti
IV
>Ti
III
10 ns (deep trap,
irreversible)
(Eq. 3.4)
Charge-Carrier Recombination
e
cb
-
+ {>Ti
IV
OH•}
+
>Ti
IV
OH
100 ns (slow)
(Eq. 3.5)
h
b
+
+ {>Ti
III
OH} Ti
IV
OH
10 ns (fast)
(Eq. 3.6)
Interfacial Charge Transfer
{>Ti
IV
OH•}-h
tr
+
+ Red
ads
> Ti
IV
OH + Red
ads
100 ns (slow)
(Eq. 3.7)
{>Ti
III
OH}-e
tr
-
+ Ox
ads
> Ti
IV
OH + Ox·
-
ads
ms (very slow)
(Eq. 3.8)
Note that >Ti
IV
OH represents the primary hydrated surface functionality of TiO
2
,
e
cb
-
is a conduction-band electron, h
b
+
is a valence-band hole, Red
ads
is an electron
donor adsorbed on the surface (i.e. reductant), Ox
ads
is an electron acceptor adsorbed on
the surface, (i.e., oxidant), {>Ti
III
OH}-e
tr
-
is the surface trapped conduction-band
electron and {>Ti
IV
OH
•
}
+
-h
tr
+
is the surface trapped valance band holes. Figure 3.4
shows the conceptualized illustration of the above reactions.
Most of the adsorbed organic pollutants (Red
ads
) can be directly oxidized by
trapped electron holes,
h
tr
+
at the TiO
2
surface. One the contrary, in the conduction-band,
the adsorbed molecular can be reduced directly by the trapped electron, i.e.
e
tr
-
, and
produce anionic radicals Ox·
-
ads
(Hoffmann et al., 1995). Here, it is assumed that the
substrates do not undergo direct hole transfer and electron reduction, as only a negligible
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