Topics in Current Chemistry (2020) 378:6
1 3
Sacrificial hydrogen production over magnetite was reported by Mangrulkar and
co-workers in the presence of ethanol as sacrificial reagent, with platinum as cocatalyst, and under UVA–Vis (tungsten filament lamp) irradiation [142]. The material was synthesized in the form of 10–12 nm nanoparticles, and the bandgap was
roughly estimated from the UV–Vis spectrum as 2.7 eV, considerably larger than the
value of ca. 1.9 eV determined from photoelectrochemical measurements [143]. The
Pt/Fe 3 O 4 photocatalytic system was active for hydrogen evolution in the presence of
different sacrificial reagents, with ethanol giving the best results over methanol and
ethanolamine. Experiments carried out in different conditions revealed that hydrogen evolved only above a certain value of irradiation power, and that, in the dark,
there was hydrogen evolution when the temperature was raised. This suggests that
the reaction has a thermal component, and indeed experiments under photothermal
conditions revealed a synergistic effect of the photocatalytic and the thermocatalytic effects, even at temperatures as low as 100 °C. Deactivation of the catalyst was
observed in all cases, and photothermal conditions also enhanced this deactivation.
Gobara et al. compared the activity of magnetite for hydrogen production from
methanol aqueous solutions under simulated solar radiation with those of different
ferrites resulting from the substitution of several divalent cations (Zn
2+
, Cd
2+
or
Ni
2+
) for Fe
2+
[144]. All catalysts were prepared by co-precipitation methods which
resulted in nanocrystalline cubic phases except for the cadmium ferrite which was
considerably less crystalline and contained a segregated Fe 2 O 3 phase. The bandgaps
decreased upon metal substitution from 2.8 eV in Fe 3 O 4 to 1.62, 1.90 and 2.23 eV
in Ni, Cd and Zn ferrites, respectively. All samples gave rise to hydrogen evolution
without the use of any co-catalyst and with stable rates along 72 h. All three ferrites led to higher hydrogen evolution rates than magnetite in the order Zn < Cd < Ni,
although in the case of the cadmium sample this should be viewed cautiously
because of the presence of the mentioned iron oxide phase. The superior activity
of nickel ferrite was attributed to the affinity of Ni for hydrogen. Nonetheless, the
NiFe 2 O 4 sample displayed the highest metal surface area as determined by hydrogen
chemisorption.
Indeed, NiFe 2 O 4 is probably the most extensively explored ferrite photocatalyst
for water splitting reactions. As summarized in Table 5, some works have reported
its activity in different experimental conditions, with a quite wide range of hydrogen
production rates. An apparent quantum yield of 0.53% was obtained by Rekhila et al.
with a sol–gel-prepared nickel ferrite using thiosulfate as sacrificial reagent without
any co-catalyst, under the full spectrum of a halogen lamp [127] and under optimum conditions of pH, S 2 O 3
2−
concentration and catalyst mass. An electrochemical
impedance spectroscopy analysis revealed the p-type semiconductivity of NiFe 2 O 4 ,
as well as the drastic increase of the lifetime of electrons under illumination in the
presence of the sacrificial reagent, emphasizing its crucial role in improving the
charge separation by hole scavenging. Peng and co-workers obtained NiFe 2 O 4 nanoparticles with homogeneous size distribution and high surface area by means of a
surfactant-assisted hydrothermal method [145]. These nanoparticles gave rise, under
visible light (λ ≥ 420 nm) and in the absence of any co-catalyst, to hydrogen evolution from an aqueous methanol solution with an apparent quantum yield of 0.52%,
after an optimum calcination temperature of 500 °C in which a compromise between
136
Reprinted from the journal
1 3
Sacrificial hydrogen production over magnetite was reported by Mangrulkar and
co-workers in the presence of ethanol as sacrificial reagent, with platinum as cocatalyst, and under UVA–Vis (tungsten filament lamp) irradiation [142]. The material was synthesized in the form of 10–12 nm nanoparticles, and the bandgap was
roughly estimated from the UV–Vis spectrum as 2.7 eV, considerably larger than the
value of ca. 1.9 eV determined from photoelectrochemical measurements [143]. The
Pt/Fe 3 O 4 photocatalytic system was active for hydrogen evolution in the presence of
different sacrificial reagents, with ethanol giving the best results over methanol and
ethanolamine. Experiments carried out in different conditions revealed that hydrogen evolved only above a certain value of irradiation power, and that, in the dark,
there was hydrogen evolution when the temperature was raised. This suggests that
the reaction has a thermal component, and indeed experiments under photothermal
conditions revealed a synergistic effect of the photocatalytic and the thermocatalytic effects, even at temperatures as low as 100 °C. Deactivation of the catalyst was
observed in all cases, and photothermal conditions also enhanced this deactivation.
Gobara et al. compared the activity of magnetite for hydrogen production from
methanol aqueous solutions under simulated solar radiation with those of different
ferrites resulting from the substitution of several divalent cations (Zn
2+
, Cd
2+
or
Ni
2+
) for Fe
2+
[144]. All catalysts were prepared by co-precipitation methods which
resulted in nanocrystalline cubic phases except for the cadmium ferrite which was
considerably less crystalline and contained a segregated Fe 2 O 3 phase. The bandgaps
decreased upon metal substitution from 2.8 eV in Fe 3 O 4 to 1.62, 1.90 and 2.23 eV
in Ni, Cd and Zn ferrites, respectively. All samples gave rise to hydrogen evolution
without the use of any co-catalyst and with stable rates along 72 h. All three ferrites led to higher hydrogen evolution rates than magnetite in the order Zn < Cd < Ni,
although in the case of the cadmium sample this should be viewed cautiously
because of the presence of the mentioned iron oxide phase. The superior activity
of nickel ferrite was attributed to the affinity of Ni for hydrogen. Nonetheless, the
NiFe 2 O 4 sample displayed the highest metal surface area as determined by hydrogen
chemisorption.
Indeed, NiFe 2 O 4 is probably the most extensively explored ferrite photocatalyst
for water splitting reactions. As summarized in Table 5, some works have reported
its activity in different experimental conditions, with a quite wide range of hydrogen
production rates. An apparent quantum yield of 0.53% was obtained by Rekhila et al.
with a sol–gel-prepared nickel ferrite using thiosulfate as sacrificial reagent without
any co-catalyst, under the full spectrum of a halogen lamp [127] and under optimum conditions of pH, S 2 O 3
2−
concentration and catalyst mass. An electrochemical
impedance spectroscopy analysis revealed the p-type semiconductivity of NiFe 2 O 4 ,
as well as the drastic increase of the lifetime of electrons under illumination in the
presence of the sacrificial reagent, emphasizing its crucial role in improving the
charge separation by hole scavenging. Peng and co-workers obtained NiFe 2 O 4 nanoparticles with homogeneous size distribution and high surface area by means of a
surfactant-assisted hydrothermal method [145]. These nanoparticles gave rise, under
visible light (λ ≥ 420 nm) and in the absence of any co-catalyst, to hydrogen evolution from an aqueous methanol solution with an apparent quantum yield of 0.52%,
after an optimum calcination temperature of 500 °C in which a compromise between
136
Reprinted from the journal