Environmental Engineering Reference
In-Depth Information
of photochemical reactions are described as follows:
4
S
∗
4
S
+
4
hγ
=
(9.2.13)
4
S
∗
+
4
S
+
+
4(
C
R
)
∗
4
C
R
=
(9.2.14)
4(
C
R
)
∗
+
4
H
+
=
4H
∗
4
C
R
+
(9.2.15)
4
S
+
+
4
C
OX
4
C
OX
=
4
S
+
(9.2.16)
4
C
OX
+
2O
∗
+
4
H
+
2
H
2
O
=
4
C
OX
+
(9.2.17)
4H
∗
=
2H
2
(9.2.18)
2O
∗
=
O
2
(9.2.19)
10
−
34
J
where
h
is the Planck constant (6.626
s),
γ
is the photon frequency, and
their product means a photon and its energy. Also,
S
means sensitizer and
C
R
and
C
OX
indicate the catalysts for the reduction and oxidization reactions, respectively. The
superscript asterisk means the activated state. If viewed from the reaction mechanism
shown in Equations 9.2.13-9.2.19, photochemical, photodissociation, photodecom-
position, photodegradation, photocatalytic, and photolysis can be categorized as the
same type. The reaction mechanism may suggest very different engineering approaches,
compared with the heat and electric potential driven water spitting, which will be
discussed in later sections.
In this type of water splitting processes, the performance of the sensitizers and
catalysts is often assessed with photon-use efficiency, which is defined on the basis of the
absorbed photons (Melis 2004), which is often adopted for describing the efficiency of
the photocatalytic reactions. Another useful efficiency is called “quantum efficiency'',
which is defined as the ratio of the number of charge carriers collected by a solar
cell to the number of photons illuminating on the solar cell (Park et al., 2009). This
efficiency is often adopted to evaluate the yield of incident photon to charge carriers
for photovoltaic panels. Since photon energy varies with wavelength, consequently
the quantum efficiency may vary for different wavelengths of light. There are also
definitions on the values of enthalpy and Gibbs free energy (Rajeshwar et al., 2008) but
in units of ev/molecule from the molecular level. Their values are shown in Equations
9.2.3 and 9.2.5, i.e., 2.97 ev/molecule and 2.47 ev/molecule for energy balance and
spontaineity threshhold, respectively.
As discussed in the previous sections, an electrode must be utilized to cre-
ate sufficient potential and sunlight must be converted to electric current in the
water electrolysis and photoelectrolysis. If an electrode is not needed, then the
terminologies “photochemical'', “photocatalytic'', “photodissociation'', “photode-
composition'', and “photolysis'' can be regarded to have a common engineering setup
and similar reaction mechanisms that do not utilize the electric potential to break the
chemical bond of hydrogen and oxygen. If there is no electrode, the energy of the pho-
tons must be absorbed and stored in some intermediate reagents and then delivered by
the reagent to water molecules. Water is transparent to a large portion of the photons
in the terrestrial solar spectrum and the photons cannot be directly utilized to break
the hydrogen and oxygen bond. Firstly, a reagent is needed that must have the abil-
ity to serve as a photon sensitizer to absorb photons and use the photons to activate
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