Topics in Current Chemistry (2020) 378:3
1 3
description of the main harmful reagents and energy-consuming processes of
materials tested in photocatalytic reactions are summarized in Table 1.
In this regard, it is worth highlighting mechanochemistry as a highly promising methodology, which in addition to its remarkable simplicity, reproducibility,
and versatility, reduces or eliminates the need to use additional reagents and solvents, thus rendering it not only economically efficient (including the short reactions times), but also—and more importantly—highly sustainable [86].
There is a long way to go, starting with the problems that are associated
with the photocatalytic process itself. The relatively low photonic and quantum
efficiency obtained for a wide range of photoreactions, together with the low
absorption of the more active pure photocatalysts such as TiO 2 or ZnO (and any
advanced material), remain the main drawback of the technology [5]. The development of samples containing minor entities such as iron, copper, cerium oxides,
or boron can help in managing the optical properties of the samples in order to
improve the final efficiency of the process. However, this is a relatively difficult
task considering that in many cases, the wastes used as raw materials have fixed
concentrations of relevant elements, which leaves little flexibility in the design of
the controlled structures.
The large-scale production of waste-derived catalytic materials must also be
further developed. The electronic and automotive industries or galvanizing plants
produce enough waste for large-scale catalyst/photocatalyst production [98].
However, most of the reports in the literature provide information only about
basic laboratory studies. As photocatalysis is not an established large-scale process, the final structure/properties of the samples is not a defined variable. Activity, selectivity, and stability must be tested using such waste-derived materials
under large-scale operating conditions. With regard to the synthetic protocols, it
is well known that the formation of precipitates must be avoided, and other processes such as filtration should be as rapid as possible; therefore, some of the
aforementioned synthetic methods must be optimized for large-scale material production. It is expected that in the coming years, life-cycle and techno-economic
assessment of the most promising photocatalytic processes will provide relevant
information about the suitability of waste-derived photocatalysts.
Although these issues are not limited to waste-derived photocatalysts, a comprehensive understanding of the processes occurring in these types of materials is
certainly lacking. In situ characterization under real-world operating conditions
must be undertaken in order to provide a clear description of entities participating in the photoreaction. In situ X-ray absorption spectroscopy (XAS), XPS, or
diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies
(under illumination conditions), in combination with modeling and analysis of
the optical properties, could prove useful in the interpretation of information
related to the mechanistic and photo-handling process [13, 99]. Notwithstanding
all the advances in the utilization of waste-derived materials as photocatalysts,
the remarkable potential of these materials can lead to even further possibilities.
We hope that this contribution can inspire the scientific community involved in
the field of photocatalysis to take full advantage of the benefits offered by waste
22
Reprinted from the journal
1 3
description of the main harmful reagents and energy-consuming processes of
materials tested in photocatalytic reactions are summarized in Table 1.
In this regard, it is worth highlighting mechanochemistry as a highly promising methodology, which in addition to its remarkable simplicity, reproducibility,
and versatility, reduces or eliminates the need to use additional reagents and solvents, thus rendering it not only economically efficient (including the short reactions times), but also—and more importantly—highly sustainable [86].
There is a long way to go, starting with the problems that are associated
with the photocatalytic process itself. The relatively low photonic and quantum
efficiency obtained for a wide range of photoreactions, together with the low
absorption of the more active pure photocatalysts such as TiO 2 or ZnO (and any
advanced material), remain the main drawback of the technology [5]. The development of samples containing minor entities such as iron, copper, cerium oxides,
or boron can help in managing the optical properties of the samples in order to
improve the final efficiency of the process. However, this is a relatively difficult
task considering that in many cases, the wastes used as raw materials have fixed
concentrations of relevant elements, which leaves little flexibility in the design of
the controlled structures.
The large-scale production of waste-derived catalytic materials must also be
further developed. The electronic and automotive industries or galvanizing plants
produce enough waste for large-scale catalyst/photocatalyst production [98].
However, most of the reports in the literature provide information only about
basic laboratory studies. As photocatalysis is not an established large-scale process, the final structure/properties of the samples is not a defined variable. Activity, selectivity, and stability must be tested using such waste-derived materials
under large-scale operating conditions. With regard to the synthetic protocols, it
is well known that the formation of precipitates must be avoided, and other processes such as filtration should be as rapid as possible; therefore, some of the
aforementioned synthetic methods must be optimized for large-scale material production. It is expected that in the coming years, life-cycle and techno-economic
assessment of the most promising photocatalytic processes will provide relevant
information about the suitability of waste-derived photocatalysts.
Although these issues are not limited to waste-derived photocatalysts, a comprehensive understanding of the processes occurring in these types of materials is
certainly lacking. In situ characterization under real-world operating conditions
must be undertaken in order to provide a clear description of entities participating in the photoreaction. In situ X-ray absorption spectroscopy (XAS), XPS, or
diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies
(under illumination conditions), in combination with modeling and analysis of
the optical properties, could prove useful in the interpretation of information
related to the mechanistic and photo-handling process [13, 99]. Notwithstanding
all the advances in the utilization of waste-derived materials as photocatalysts,
the remarkable potential of these materials can lead to even further possibilities.
We hope that this contribution can inspire the scientific community involved in
the field of photocatalysis to take full advantage of the benefits offered by waste
22
Reprinted from the journal