Topics in Current Chemistry (2020) 378:6
1 3
a single dual catalyst allowed La 1–x Ti x FeO 3 to simultaneously take advantage of the
higher reaction rate of the photoassisted heterogeneous Fenton process and of the
higher mineralization yield of photocatalysis. This Ti-substituted LaFeO 3 dual catalyst was reported to be very active in water treatment, by reaching complete mineralization and total pollutant removal with higher efficiencies than via single AOPs,
whether the process operates under UVA light, simulated solar light or pure visible
light (λ > 420 nm). The dual catalyst operates via pure heterogeneous surface reactions with an increase in the catalyst robustness by more than two orders of magnitude compared to the unsubstituted ferrite. This was symbolized by the absence of
any Fe release and by the stability of catalytic performance with test cycles with no
loss of activity [106].
3 Ferrite‑Based Photocatalysts and Photoelectrodes for Energy
Applications: Hydrogen Production and CO 2 Reduction
3.1 Introduction
Energy applications of photocatalysis, mainly CO 2 reduction and hydrogen production by water splitting or photoreforming, are technologically less developed than
environmental ones (viz. pollutant degradation), although their scientific interest has
increasingly grown in the last decades as a result of their appealing possibilities in a
hopefully not-so-long-term circular economy based on solar energy [107].
The production of hydrogen from renewable feedstock and energy sources is crucial for its implementation as a clean energy vector in a future non-carbon-based
energy system [108]. Photocatalysis and photoelectrochemistry offer the possibility
of directly converting sunlight into chemical energy by splitting the water molecule
into hydrogen and oxygen, making use at the surface of semiconductor photocatalysts of the reductive and oxidative power of photo-generated electrons and holes,
respectively. Alternatively, as a result of the kinetic constraints of water oxidation,
hydrogen can be produced using organic sacrificial agents, ideally derived from biomass feedstock, in the process called photoreforming [109, 110]. In the last decades,
vast amounts of semiconductors have been tested as photocatalysts for photocatalytic or photoelectrochemical water splitting [107, 111, 112], among which ironbased oxides and particularly ferrites have been less explored than other types of
materials. This is generally related to low charge carrier mobilities and hence short
exciton lifetimes, which nevertheless can be improved by doping or by other chemical, structural or surface modifications [113, 114]. Some ferrites indeed have the
adequate band positions for at least one of the half-reactions (proton reduction and
water oxidation) involved in water splitting and, furthermore, these positions can be
tuned by means of the inclusion of different metals into the ferrite structure [114].
As described below, some ferrites have been reported active for photocatalytic
hydrogen evolution. Selected examples are summarized in Table 5.
Regarding photoelectrochemical hydrogen production, varied chemical composition, multiple valence states and choice of metal cation, narrow bandgaps and both
n-type and p-type behaviour convert ferrites into attractive candidates as electrodes
132
Reprinted from the journal
1 3
a single dual catalyst allowed La 1–x Ti x FeO 3 to simultaneously take advantage of the
higher reaction rate of the photoassisted heterogeneous Fenton process and of the
higher mineralization yield of photocatalysis. This Ti-substituted LaFeO 3 dual catalyst was reported to be very active in water treatment, by reaching complete mineralization and total pollutant removal with higher efficiencies than via single AOPs,
whether the process operates under UVA light, simulated solar light or pure visible
light (λ > 420 nm). The dual catalyst operates via pure heterogeneous surface reactions with an increase in the catalyst robustness by more than two orders of magnitude compared to the unsubstituted ferrite. This was symbolized by the absence of
any Fe release and by the stability of catalytic performance with test cycles with no
loss of activity [106].
3 Ferrite‑Based Photocatalysts and Photoelectrodes for Energy
Applications: Hydrogen Production and CO 2 Reduction
3.1 Introduction
Energy applications of photocatalysis, mainly CO 2 reduction and hydrogen production by water splitting or photoreforming, are technologically less developed than
environmental ones (viz. pollutant degradation), although their scientific interest has
increasingly grown in the last decades as a result of their appealing possibilities in a
hopefully not-so-long-term circular economy based on solar energy [107].
The production of hydrogen from renewable feedstock and energy sources is crucial for its implementation as a clean energy vector in a future non-carbon-based
energy system [108]. Photocatalysis and photoelectrochemistry offer the possibility
of directly converting sunlight into chemical energy by splitting the water molecule
into hydrogen and oxygen, making use at the surface of semiconductor photocatalysts of the reductive and oxidative power of photo-generated electrons and holes,
respectively. Alternatively, as a result of the kinetic constraints of water oxidation,
hydrogen can be produced using organic sacrificial agents, ideally derived from biomass feedstock, in the process called photoreforming [109, 110]. In the last decades,
vast amounts of semiconductors have been tested as photocatalysts for photocatalytic or photoelectrochemical water splitting [107, 111, 112], among which ironbased oxides and particularly ferrites have been less explored than other types of
materials. This is generally related to low charge carrier mobilities and hence short
exciton lifetimes, which nevertheless can be improved by doping or by other chemical, structural or surface modifications [113, 114]. Some ferrites indeed have the
adequate band positions for at least one of the half-reactions (proton reduction and
water oxidation) involved in water splitting and, furthermore, these positions can be
tuned by means of the inclusion of different metals into the ferrite structure [114].
As described below, some ferrites have been reported active for photocatalytic
hydrogen evolution. Selected examples are summarized in Table 5.
Regarding photoelectrochemical hydrogen production, varied chemical composition, multiple valence states and choice of metal cation, narrow bandgaps and both
n-type and p-type behaviour convert ferrites into attractive candidates as electrodes
132
Reprinted from the journal