The addition of the oxidant species exhibits a dual function: acting as strong
oxidants, they will tend to increase the formation of hydroxyl radicals but also inhibit
the electron/hole (e
À
/h+) pair recombination; however, all the oxidizing agents do
not have the same impact on photocatalytic degradation. Indeed, among those cited
above, researchers have shown that H 2 O 2 is generally the most effective oxidant
(Ahmed et al. 2010). In that case, the enhancement of the degradation can be
explained as follows (Akpan et al):
H 2 O 2 þ
• O
À
2 !
• OH þ OH
À
þ O 2
H 2 O 2 þ hv ! 2
• OH
H 2 O 2 þ e
À
!
• OH þ OH
À
This effect of H 2 O 2 has been demonstrated by Chiou et al. in the photocatalytic
degradation of phenol where the increase of the removal efficiency is observed (58 to
84% within 3 h with the addition of H 2 O 2 from 1.77 mM to 8.82 mM) (Chiou et al.
2008). In this study, they also show the total degradation of phenol within 2.5 h and
1h by increasing the H 2 O 2 concentration from 44.1 mM to 88.2 mM.
In addition, it has been generally shown that the UV/TiO 2 /oxidant degradation
process is more effective in acidic than in basic medium (Ahmed et al. 2010).
Some studies, however, have shown that an optimal concentration of H 2 O 2
should be taken into account. In fact, at higher concentrations, for example, the
degradation of orange G on N-doped TiO 2 decrease (Akpan and Hameed 2009). This
can be explained by the consumption of
•
OH radicals and holes by H 2 O 2 itself as
described as follows (Mahmoodi et al. 2006; Coleman et al. 2007):
• OH þ H 2 O 2 ! HO
•
2 þ H 2 O
• OH þ HO
•
2 ! O 2 þ H 2 O
h
þ
þ H 2 O 2 ! O 2 þ 2H
þ
In parallel, the recombination of radicals must be taken into account as a
competitive reaction that can occur:
• OHþ
• OH ! H 2 O 2
Both
•
OH and h
+ can finally be considered as strong oxidants for organic
pollutants, but in the excess of H 2 O 2 concentration, the photocatalytic degradation
can be inhibited. Moreover, H 2 O 2 can be adsorbed on TiO 2 particles that leads to
surface modification and thus to the photocatalytic degradation (Tanaka et al. 1989).
However, in various studies many authors have reported adsorption of compounds or intermediates that may acts as poison on the catalyst surface, e.g., a
multilayer of dye molecules around the catalyst particles surface which leads to
limited interaction between excited dye molecule and the catalyst and thus a decrease
in photooxidation process (Bizani et al. 2006).
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
51
oxidants, they will tend to increase the formation of hydroxyl radicals but also inhibit
the electron/hole (e
À
/h+) pair recombination; however, all the oxidizing agents do
not have the same impact on photocatalytic degradation. Indeed, among those cited
above, researchers have shown that H 2 O 2 is generally the most effective oxidant
(Ahmed et al. 2010). In that case, the enhancement of the degradation can be
explained as follows (Akpan et al):
H 2 O 2 þ
• O
À
2 !
• OH þ OH
À
þ O 2
H 2 O 2 þ hv ! 2
• OH
H 2 O 2 þ e
À
!
• OH þ OH
À
This effect of H 2 O 2 has been demonstrated by Chiou et al. in the photocatalytic
degradation of phenol where the increase of the removal efficiency is observed (58 to
84% within 3 h with the addition of H 2 O 2 from 1.77 mM to 8.82 mM) (Chiou et al.
2008). In this study, they also show the total degradation of phenol within 2.5 h and
1h by increasing the H 2 O 2 concentration from 44.1 mM to 88.2 mM.
In addition, it has been generally shown that the UV/TiO 2 /oxidant degradation
process is more effective in acidic than in basic medium (Ahmed et al. 2010).
Some studies, however, have shown that an optimal concentration of H 2 O 2
should be taken into account. In fact, at higher concentrations, for example, the
degradation of orange G on N-doped TiO 2 decrease (Akpan and Hameed 2009). This
can be explained by the consumption of
•
OH radicals and holes by H 2 O 2 itself as
described as follows (Mahmoodi et al. 2006; Coleman et al. 2007):
• OH þ H 2 O 2 ! HO
•
2 þ H 2 O
• OH þ HO
•
2 ! O 2 þ H 2 O
h
þ
þ H 2 O 2 ! O 2 þ 2H
þ
In parallel, the recombination of radicals must be taken into account as a
competitive reaction that can occur:
• OHþ
• OH ! H 2 O 2
Both
•
OH and h
+ can finally be considered as strong oxidants for organic
pollutants, but in the excess of H 2 O 2 concentration, the photocatalytic degradation
can be inhibited. Moreover, H 2 O 2 can be adsorbed on TiO 2 particles that leads to
surface modification and thus to the photocatalytic degradation (Tanaka et al. 1989).
However, in various studies many authors have reported adsorption of compounds or intermediates that may acts as poison on the catalyst surface, e.g., a
multilayer of dye molecules around the catalyst particles surface which leads to
limited interaction between excited dye molecule and the catalyst and thus a decrease
in photooxidation process (Bizani et al. 2006).
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
51