302
Water for Energy and Fuel Production
heterogeneous photoproduction of hydrogen and oxygen from water. The majority
of the photoredox systems (heterogeneous photolysis) involve a photosensitizer, an
electron acceptor, and an electron donor, with the redox catalyst assisting in the gas
evolution step. Excitation of the sensitizer (S) leads to an electron transfer:
S + A hv S + A
(11.7)
which is followed by the catalytic step:
A
−
1
+ H 2 O cat A + OH
−
+ H
2
2
(11.8)
The back conversion of S + to S may be achieved by sacrificing a donor D added to
the solution
S
+
+ D
−
→ S + D
+
(11.9)
Koriakin et al. [39] used acridine dyes as sensitizers, Eu 3+ , V 2+ salicylates as electron
acceptors, and “Adams” catalyst (PtO 2 ) as the redox catalysts. Numerous other sensitizers, electron acceptors, and redox catalysts are illustrated by Bockris et al. [11]. An
efficiency of up to 30% at an elected wavelength for hydrogen production for a brief
duration by photolytic process has been reported by Kalyanasundaram et al. [40].
11.3.1 WATer SPliTTing on SemiConduCTor CATAlySTS (PhoToCATAlySiS)
Duonghong et al. [41] were the first to investigate the splitting of water by utilizing microsystems [41–65] (Correa, 2009, pers. comm.). In this system, the colloidal particles are made up of suitable conductor materials, for example, TiO 2 . On
these colloids are induced two metallic substances, for example, ruthenium oxide
and platinum. When the system is irradiated, hydrogen is evolved on the platinum
and oxygen on ruthenium oxide. Each colloidal particle is a micro photocell. Using
small TiO 2 particles, a large area of TiO 2 can be exposed to light. The system needs
to be heated to last more than several hours. There are some doubts whether or not
equal production of hydrogen and oxygen is achieved and whether oxygen is engaged
in side reactions. It is difficult to measure the efficiency of this system. In addition,
hydrogen and oxygen come off from water together and their separations add extra
cost. Furthermore, the simultaneous presence of oxygen and hydrogen in water can
give rise to chemical catalysis and recombination to water [41–65].
11.3.1.1 titanium Oxide Photocatalysts
TiO 2 was the first semiconductor used in water dissociation reaction [56]. A pure
and powdered TiO 2 , however, only absorbs UV fraction of solar light and thus not
very effective for total absorption of solar light. The visible light response of TiO 2
was improved by chemical doping of TiO 2 with partially filled d-orbitals such as V 5
+
,
+
+
+
Cr 3 , Fe 3 , CO 2 , and Ni 2
+ [44,61]. While these doping improved visible light response,
they did not improve water dissociation reaction. Kato, Kudo, and coworkers [57–61]
reported that TiO 2 co-doped with a combination of Sb 5
+ and Cr 3
+ became active for
O 2 evolution under visible light from an aqueous solution using AgNO 3 as sacrificial
Water for Energy and Fuel Production
heterogeneous photoproduction of hydrogen and oxygen from water. The majority
of the photoredox systems (heterogeneous photolysis) involve a photosensitizer, an
electron acceptor, and an electron donor, with the redox catalyst assisting in the gas
evolution step. Excitation of the sensitizer (S) leads to an electron transfer:
S + A hv S + A
(11.7)
which is followed by the catalytic step:
A
−
1
+ H 2 O cat A + OH
−
+ H
2
2
(11.8)
The back conversion of S + to S may be achieved by sacrificing a donor D added to
the solution
S
+
+ D
−
→ S + D
+
(11.9)
Koriakin et al. [39] used acridine dyes as sensitizers, Eu 3+ , V 2+ salicylates as electron
acceptors, and “Adams” catalyst (PtO 2 ) as the redox catalysts. Numerous other sensitizers, electron acceptors, and redox catalysts are illustrated by Bockris et al. [11]. An
efficiency of up to 30% at an elected wavelength for hydrogen production for a brief
duration by photolytic process has been reported by Kalyanasundaram et al. [40].
11.3.1 WATer SPliTTing on SemiConduCTor CATAlySTS (PhoToCATAlySiS)
Duonghong et al. [41] were the first to investigate the splitting of water by utilizing microsystems [41–65] (Correa, 2009, pers. comm.). In this system, the colloidal particles are made up of suitable conductor materials, for example, TiO 2 . On
these colloids are induced two metallic substances, for example, ruthenium oxide
and platinum. When the system is irradiated, hydrogen is evolved on the platinum
and oxygen on ruthenium oxide. Each colloidal particle is a micro photocell. Using
small TiO 2 particles, a large area of TiO 2 can be exposed to light. The system needs
to be heated to last more than several hours. There are some doubts whether or not
equal production of hydrogen and oxygen is achieved and whether oxygen is engaged
in side reactions. It is difficult to measure the efficiency of this system. In addition,
hydrogen and oxygen come off from water together and their separations add extra
cost. Furthermore, the simultaneous presence of oxygen and hydrogen in water can
give rise to chemical catalysis and recombination to water [41–65].
11.3.1.1 titanium Oxide Photocatalysts
TiO 2 was the first semiconductor used in water dissociation reaction [56]. A pure
and powdered TiO 2 , however, only absorbs UV fraction of solar light and thus not
very effective for total absorption of solar light. The visible light response of TiO 2
was improved by chemical doping of TiO 2 with partially filled d-orbitals such as V 5
+
,
+
+
+
Cr 3 , Fe 3 , CO 2 , and Ni 2
+ [44,61]. While these doping improved visible light response,
they did not improve water dissociation reaction. Kato, Kudo, and coworkers [57–61]
reported that TiO 2 co-doped with a combination of Sb 5
+ and Cr 3
+ became active for
O 2 evolution under visible light from an aqueous solution using AgNO 3 as sacrificial