Environmental Engineering Reference
In-Depth Information
Table 13.4.1
Optical properties for some photocatalytic semiconducting
materials.
Band gap equivalent
Semiconductor
Band gap (eV)
wavelength (nm)
BaTiO
3
3.3
375
CdO
2.1
590
CdS
2.5
497
CdSe
1.7
730
Fe
2
O
3
2.2
565
GaS
1.4
887
GaP
2.3
540
SnO
2
3.9
318
SrTiO
3
3.4
365
TiO
2
3.0
390
WO
3
2.8
443
ZnO
3.2
390
ZnS
3.7
336
TiO
2
has proven to be highly effective in the nonselective decomposition of organic
molecules and inactivation of microorganisms due to high decomposition and miner-
alization rates when used in combination with specific radiation sources (see Table
13.4.1). Furthermore, water treatment with SDHP neither requires the addition of
other chemicals reactants nor generates hazardous waste by-products.
The basic reactions occurring within the photocatalyst particles after absorption
of the proper wavelength radiation are as follows:
hv
→
e
−
+
h
+
+
TiO
2
TiO
2
(13.4.1)
e
−
+
h
+
+
hv
TiO
2
→
TiO
2
+
(13.4.2)
(TiO
IV
Ti
IV
)
h
BV
→
(TiO
IV
Ti
IV
)
H
+
−
O
2
−
−
−
OH
+
−
O
2
−
−
−
OH
+
e
BC
→
O
·
2(
ads
)
+
O
2(
ads
)
(13.4.3)
As shown in Equations 13.4.1 to 13.4.4, several oxidizing species may be gen-
erated as a result of the interaction between the photocatalyst and the radiation,
although evidence supports the idea that the hydroxyl radical (
·
OH) is the main oxi-
dizing specie responsible for the photo-oxidation of most organic compounds studied.
Once the absorption of one photon with the required energy (ultraviolet radiation,
λ<
390 nm) occurs, the first step is the generation of electron/hole pairs, which are
separated between the conduction and valence bands (Equation 13.4.1). Recombina-
tion of the generated charge carriers (Equation 13.4.2) may occur. However, if the
dissolvent is redox active (i.e. water) it may acts as a donor and acceptor of electrons
avoiding recombination and improving the photonic efficiency, producing that on a
hydrated and hydroxylated TiO
2
surface, the holes trap
·
OH radicals linked to the
surface and avoid recombination reactions (Equation 13.4.3). It should be emphasized
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