304
Water for Energy and Fuel Production
oxynitride photocatalysts for pure water dissociation in visible light [44]. Finally,
(Zn 1 x Ge)(N 2 O x ) solid solution loaded with nanoparticulate RuO 2 cocatalyst is also
+
active under visible light, generating hydrogen and oxygen stoichiometrically from
pure water [44].
11.3.1.4 metal sulfides
Navarro et al. [44] reported that while small band gaps in metal sulfides make them
very attractive photocatalysts for water dissociation, they are unstable for water oxidation reaction under visible light. A common method for the reducing photocorrosion of the sulfides under irradiation is the use of suitable sacrificial agents such as
Na 2 S/Na 2 SO 3 salt mixture [44]. CdS with wurtzite structure is the best-studied metal
sulfide photocatalyst [66–68]. This catalyst property can be improved by improving preparation method that leads to CdS phases with good crystallinity and few
crystal defects. Composite systems of CdS with TiO 2 , ZnO, and CdO [69–71] also
improved photoactivity. The incorporation of elements into the structure of CdS to
make a solid solution is another strategy for improving the photocatalytic properties
of CdS [44]. The substitution of ZnS into CdS structure improved the activity of the
composite material [44].
ZnS was also another semiconductor investigated for photocatalytic activity [44].
The chemical doping of ZnS by Cu
+
2 , Ni
+
2 , and Pb
+
2 [59,60,72,73] allowed ZnS to
absorb visible light. These doped ZnS photocatalysts showed high photocatalytic
activity under visible light for hydrogen production from aqueous solutions using
SO
2−
2
3
S as electron donor reagents. Combining ZnS with AglnS 2 and CulnS 2 to
produce solid solutions (CuAgln) x Zn 2(1 x) S 2 is another strategy for improving opti−
cal absorption in the visible light range [44,74–76]. Co catalysts such as Pt loaded
on (Agln) 0.22 Zn 1.56 S 2 showed the highest activity for hydrogen evolution [44]. The
ternary sulfides comprising ln 3+ and one type of transition metal cation (Cd
+
, Zn
+
,
2
2
Mn
+
+
2 , Cu ) found to have low efficiency for water dissociation in visible light. More
description of sulfide photocatalysts is given by Navarro et al. [44]. An overview of
recently developed photocatalysts for water splitting under visible light illumination
is also summarized by Navarro et al. [44].
11.3.2 PhoToBiologiCAl ProduCTion oF hydrogen From WATer
The water splitting can also be carried out photobiologically [49]. Biological hydrogen can be produced in an algae bioreactor. In the late 1990s, it was discovered that if
the algae are deprived of sulfur, it will switch from the production of oxygen (a normal mode of photosynthesis) to the production of hydrogen. It seems that the production is now economically feasible by the energy efficiency surpassing 7%–10% [49].
Hydrogen can be produced from water by hydrogenase-catalyzed reduction of
protons by the electrons generated from photosynthetic oxidation of water using
sunlight energy. In the recent years, use of a variety of algae to produce hydrogen
from water has been extensively investigated and reviewed [77,78]. These reviews
mention the use of sulfur deprivation with Chlamydomonas reinhardtii to improve
hydrogen production by algae [79,80]. In addition, certain polygenetic and molecular analyses were performed in green algae [81,82]. These methods, however, did
Water for Energy and Fuel Production
oxynitride photocatalysts for pure water dissociation in visible light [44]. Finally,
(Zn 1 x Ge)(N 2 O x ) solid solution loaded with nanoparticulate RuO 2 cocatalyst is also
+
active under visible light, generating hydrogen and oxygen stoichiometrically from
pure water [44].
11.3.1.4 metal sulfides
Navarro et al. [44] reported that while small band gaps in metal sulfides make them
very attractive photocatalysts for water dissociation, they are unstable for water oxidation reaction under visible light. A common method for the reducing photocorrosion of the sulfides under irradiation is the use of suitable sacrificial agents such as
Na 2 S/Na 2 SO 3 salt mixture [44]. CdS with wurtzite structure is the best-studied metal
sulfide photocatalyst [66–68]. This catalyst property can be improved by improving preparation method that leads to CdS phases with good crystallinity and few
crystal defects. Composite systems of CdS with TiO 2 , ZnO, and CdO [69–71] also
improved photoactivity. The incorporation of elements into the structure of CdS to
make a solid solution is another strategy for improving the photocatalytic properties
of CdS [44]. The substitution of ZnS into CdS structure improved the activity of the
composite material [44].
ZnS was also another semiconductor investigated for photocatalytic activity [44].
The chemical doping of ZnS by Cu
+
2 , Ni
+
2 , and Pb
+
2 [59,60,72,73] allowed ZnS to
absorb visible light. These doped ZnS photocatalysts showed high photocatalytic
activity under visible light for hydrogen production from aqueous solutions using
SO
2−
2
3
S as electron donor reagents. Combining ZnS with AglnS 2 and CulnS 2 to
produce solid solutions (CuAgln) x Zn 2(1 x) S 2 is another strategy for improving opti−
cal absorption in the visible light range [44,74–76]. Co catalysts such as Pt loaded
on (Agln) 0.22 Zn 1.56 S 2 showed the highest activity for hydrogen evolution [44]. The
ternary sulfides comprising ln 3+ and one type of transition metal cation (Cd
+
, Zn
+
,
2
2
Mn
+
+
2 , Cu ) found to have low efficiency for water dissociation in visible light. More
description of sulfide photocatalysts is given by Navarro et al. [44]. An overview of
recently developed photocatalysts for water splitting under visible light illumination
is also summarized by Navarro et al. [44].
11.3.2 PhoToBiologiCAl ProduCTion oF hydrogen From WATer
The water splitting can also be carried out photobiologically [49]. Biological hydrogen can be produced in an algae bioreactor. In the late 1990s, it was discovered that if
the algae are deprived of sulfur, it will switch from the production of oxygen (a normal mode of photosynthesis) to the production of hydrogen. It seems that the production is now economically feasible by the energy efficiency surpassing 7%–10% [49].
Hydrogen can be produced from water by hydrogenase-catalyzed reduction of
protons by the electrons generated from photosynthetic oxidation of water using
sunlight energy. In the recent years, use of a variety of algae to produce hydrogen
from water has been extensively investigated and reviewed [77,78]. These reviews
mention the use of sulfur deprivation with Chlamydomonas reinhardtii to improve
hydrogen production by algae [79,80]. In addition, certain polygenetic and molecular analyses were performed in green algae [81,82]. These methods, however, did