78
3.2 Photocatalytic Reduction of Carbon Dioxide
In the photocatalytic process, the UV or visible light is used for carbon dioxide
conversion. The latter type of reaction is attractive due to its economical cost based
on using the solar light as a source of energy to accomplish the reaction.
In general, the photocatalytic process starts when the photocatalyst, acting as a
semiconductor, absorbs sufficient amount of photon energy from the light. This
energy causes the excitation of electrons from its valence band to conduction band,
leaving a hole in the valence band. The initiated electron-hole pairs promote the
redox chemical reactions that are necessary for carbon dioxide reduction and hydrogen production via the water-splitting reaction (Linsebigler et al. 1995).
As a green application, the researches of the photocatalytic reduction of carbon
dioxide into methanol have become an on-the-spot topic because of the potential
utilization of renewable solar energy (Appel et al. 2013). As a matter of fact, the
achievement of the conversion of carbon dioxide into methanol is not the thermodynamically favorable route, which is considered as a huge challenge for the researchers. The unfavorable formation of methanol needs the transfer of six electrons in
comparison to the required two electrons that will be needed for the formation of
carbon monoxide or formic acid (Zhang et al. 2015).
3.2.1 Photocatalytic Reduction of Carbon Dioxide Using
Transition Metal Catalysts
In spite of the well-known photocatalytic activities of TiO 2 catalysts having (331)
facets towards the reduction of carbon dioxide (Truong et al. 2017), it was found
that the incorporation of CeO 2 in titanium oxide catalyst decreased the band gap of
TiO 2 from 3.19 to 2.64 eV. Such band gap is suitable for the photocatalytic reduction of carbon dioxide into methanol (Wang et al. 2013). Also, cerium oxide stabilized the anatase phase, decreased its crystallite size, and lowered the rate of
electron-hole pair recombination (Abdullah et al. 2015). Like cerium oxide, nickel
oxide that immobilized on the activated carbon fibers showed similar effects on
TiO 2 catalyst (Sharma and Lee 2017). NiO enhanced the methanol yield to be ~
493 μmol/g.h by using NiO-TiO 2 /activated carbon fiber in comparison with
~177 μmol/g.h by using Cu-C/TiO 2 catalyst (Kavil et al. 2017) and ~19 μmol/g.h by
using CeO 2 -TiO 2 catalyst (Abdullah et al. 2015) under the sunlight.
Recently, ZnIn 2 S 4 microspheres, decorated with cerium oxide (CeO 2 ), were
found to exhibit obvious photocatalytic conversion rate of 0.542 μmol.g
−1
.h
−1
. The
oxygen vacancies of the CeO 2 showed a significant efficiently for trapping the photogenerated electrons from ZnIn 2 S 4 . Also, the high surface area of the microspheres
provided numerous active reaction sites and made the reactants and products diffuse
more easily (Yang et al. 2019).
T. Zaki
3.2 Photocatalytic Reduction of Carbon Dioxide
In the photocatalytic process, the UV or visible light is used for carbon dioxide
conversion. The latter type of reaction is attractive due to its economical cost based
on using the solar light as a source of energy to accomplish the reaction.
In general, the photocatalytic process starts when the photocatalyst, acting as a
semiconductor, absorbs sufficient amount of photon energy from the light. This
energy causes the excitation of electrons from its valence band to conduction band,
leaving a hole in the valence band. The initiated electron-hole pairs promote the
redox chemical reactions that are necessary for carbon dioxide reduction and hydrogen production via the water-splitting reaction (Linsebigler et al. 1995).
As a green application, the researches of the photocatalytic reduction of carbon
dioxide into methanol have become an on-the-spot topic because of the potential
utilization of renewable solar energy (Appel et al. 2013). As a matter of fact, the
achievement of the conversion of carbon dioxide into methanol is not the thermodynamically favorable route, which is considered as a huge challenge for the researchers. The unfavorable formation of methanol needs the transfer of six electrons in
comparison to the required two electrons that will be needed for the formation of
carbon monoxide or formic acid (Zhang et al. 2015).
3.2.1 Photocatalytic Reduction of Carbon Dioxide Using
Transition Metal Catalysts
In spite of the well-known photocatalytic activities of TiO 2 catalysts having (331)
facets towards the reduction of carbon dioxide (Truong et al. 2017), it was found
that the incorporation of CeO 2 in titanium oxide catalyst decreased the band gap of
TiO 2 from 3.19 to 2.64 eV. Such band gap is suitable for the photocatalytic reduction of carbon dioxide into methanol (Wang et al. 2013). Also, cerium oxide stabilized the anatase phase, decreased its crystallite size, and lowered the rate of
electron-hole pair recombination (Abdullah et al. 2015). Like cerium oxide, nickel
oxide that immobilized on the activated carbon fibers showed similar effects on
TiO 2 catalyst (Sharma and Lee 2017). NiO enhanced the methanol yield to be ~
493 μmol/g.h by using NiO-TiO 2 /activated carbon fiber in comparison with
~177 μmol/g.h by using Cu-C/TiO 2 catalyst (Kavil et al. 2017) and ~19 μmol/g.h by
using CeO 2 -TiO 2 catalyst (Abdullah et al. 2015) under the sunlight.
Recently, ZnIn 2 S 4 microspheres, decorated with cerium oxide (CeO 2 ), were
found to exhibit obvious photocatalytic conversion rate of 0.542 μmol.g
−1
.h
−1
. The
oxygen vacancies of the CeO 2 showed a significant efficiently for trapping the photogenerated electrons from ZnIn 2 S 4 . Also, the high surface area of the microspheres
provided numerous active reaction sites and made the reactants and products diffuse
more easily (Yang et al. 2019).
T. Zaki