Environmental Engineering Reference
In-Depth Information
The SC can de-energize in different ways. A very likely but unfavorable consequence is the electron-hole recombination, a
detrimental unproductive process. Another possibility is electron transfer reactions between electrons or holes and organic or
inorganic species adsorbed on the SC surface or close to it (and not necessarily adsorbed). The SC can donate electrons,
reducing electron acceptors (A), or a hole can receive an electron from a donor species (d), which suffers oxidation. The prob-
ability of these transfer processes depends on the relative redox positions of the CB and VB in the SC in relation to the redox
potential of the adsorbed species and of the rate of competitive processes.
TiO
2
is by far the most useful SC material for photocatalytic purposes because of its exceptional optical and electronic
properties, chemical stability, nontoxicity, and low cost. The energy band gaps of the most used photocatalytic forms of TiO
2
,
anatase and rutile, are 3.23 eV (corresponding to 384 nm) and 3.02 eV (411 nm), respectively [9]. The most popular commercial
TiO
2
material for HP was originally produced by the german company degussa (now evonik) with the denomination P-25.
Some other commercial products, such as Hombikat UV100, and different products of Cristal global (PC50, PC100, PC500),
fluka, and so on, have been tested in several photocatalytic systems [13-15]. Although the data are divergent in the literature,
the redox levels of h
VB
+
−
and e
CB
photogenerated by HP in P-25 that are taken into consideration in this chapter are +2.9 and −0.3 V,
1
respectively [16].
It has been proposed that in HP carried out under normal illumination (e.g., black light or xenon lamps), the redox processes
take place through monoelectronic step reactions, due to the low frequency of photon absorption. In the case of acceptors:
−
−
Ae A
C
+→
(9.2)
The holes can directly attack the d species adsorbed on TiO
2
, or adsorbed water molecules/surface hydroxyl groups, gener-
ating HO
∙
. These radicals are highly oxidizing species with a reduction potential of +2.8V [16]:
+
+→
•
+
hDD
VB
(9.3)
ads
ads
+
(
)
→+
+
−
•
+
h OHO OH
VB
(
)
(9.4)
surf
2
ads
The flatband position of a SC
2
follows Nernstian law and is reduced in 59 mV per unit of pH [17]; thus, the driving force of
the heterogeneous redox reactions can be varied by controlling the pH.
HP oxidative reactions are usually performed in the presence of molecular oxygen, with the objective of enhancing the min-
eralization of organic compounds; e
CB
/
( )
formation, decreasing
the probability of electron-hole recombination. The value for the reduction potential for the hydroperoxyl/superoxide radical
has been reported as
E
0
−
•
•
reduce O
2
with hydroperoxyl or superoxide radical HO O
2
2
(
)
=−
•
OHO
/
005
.
V
[18]:
2
2
++→
(
)
−
+
•
−
•
Oe HOHO
CB
(
)
(9.5)
2
2
2
•
→+
2
HO HO O
(9.6)
2
2
2
2
•
−
•
−
HO O OHOO
22 2
+→+
+
(9.7)
2
−
•
−
HO e OHO
CB
22
+→+
(9.8)
This indicates that reduction of oxygen by e
C
−
would be thermodynamically feasible at low pH. At higher pH values, a low
driving force would make this reaction more difficult.
when a metal ion is present in a HP aqueous system solution, it can undergo reduction or oxidation reactions. figure 9.1
shows the reduction potential of different couples together with the energy levels for e
CB
−
and h
V
+
; the comparison of the redox
levels leads to a first approximation of the thermodynamic feasibility of transformation of the species. All couples with redox
potentials more positive than the level of e
CB
−
can be directly reduced and all couples with redox potentials less positive than
+
the level of h
VB
can be, in principle, oxidized.
1
All reduction potentials in this work are standard values versus NHe; therefore, the values correspond to pH 0 unless a different condition is specified.
2
Potential at which no excess charge exists in the SC and there is no electric field and no space charge region so that the (conduction and valence) bands are not bent.
