580
by simultaneously meeting the increasing energy demands. Many approaches such
as thermocatalytic, photocatalytic, biocatalytic, electrocatalytic, and photoelectrocatalytic have been used for conversion of CO 2 to value-added product [112].
However, the challenge in each approach is the activation of linear CO 2 molecule
due to its high stability (dissociation energy of 750 kJ mol
−1
of C=O bond) [113].
Several value-added products like formic acid, formaldehyde, methane, and methanol are possible from CO 2 reduction reaction. CO 2 reduction process requires energy
input; hence, utilization of renewable sources of energy such as sunlight is a sustainable means. In photocatalytic processes, solar energy is utilized to carry out the
chemical reaction. Therefore, heterogeneous and homogeneous photocatalytic í µí°¶í µí± 2
reduction could be a promising way to tackle the dual problem of global warming
and meeting the increasing energy demand.
4 Advantages of Photocatalytic Conversion Process over
Thermocatalytic Conversion
The CO 2 conversion process takes place at moderate reaction conditions, i.e., at
atmospheric pressure and temperature, unlike in case of thermocatalytic process
where high temperature and pressures are required. Energy source in case of photocatalytic process is solar light which is present in enormous amount. In photocatalytic route, the hydrogen source is water, while in the case of
thermocatalytic conversion process, the source of hydrogen is H 2 gas. Hence, cost
of water as a hydrogen source in photocatalytic process is lesser compared to
hydrogen gas. The total energy input cost in case of photocatalytic is far less compared to that in case of thermocatalytic conversion process. The photocatalysts
used in case of photoconversion processes possess the quality of recyclability. As
the conversion process takes place at low temperature and pressure in case of photocatalytic conversion, the number of cycles for which catalysts can be used are
more in case of photocatalyst when compared to the recyclability of the thermocatalyst. With lesser initial investment and higher duration of use, the photocatalytic process is a promising means to utilize CO 2 for conversion to value-added
chemicals.
The photocatalysts are majorly semiconductors, possessing band structure with
conduction and valance bands. The highest occupied energy band that is completely
occupied is the valance band, and the energy band from top that is unoccupied is the
conduction band. The difference between the valance and conduction band is the
band gap, and through thermal interactions, the electrons can move from valance to
conduction band and vice versa. Conduction band energy is higher compared to that
of the valance band. The population density of the charge carriers, i.e., electron
(negative charge carrier) and holes (positive charge carriers), can change upon interaction with light. For the activation of stable CO 2 molecules, the energy input is
desired; in photocatalytic process, this energy is provided by the high-energy electrons. The high-energy electrons are the electrons that are present in the conduction
K. Tripathi et al.
by simultaneously meeting the increasing energy demands. Many approaches such
as thermocatalytic, photocatalytic, biocatalytic, electrocatalytic, and photoelectrocatalytic have been used for conversion of CO 2 to value-added product [112].
However, the challenge in each approach is the activation of linear CO 2 molecule
due to its high stability (dissociation energy of 750 kJ mol
−1
of C=O bond) [113].
Several value-added products like formic acid, formaldehyde, methane, and methanol are possible from CO 2 reduction reaction. CO 2 reduction process requires energy
input; hence, utilization of renewable sources of energy such as sunlight is a sustainable means. In photocatalytic processes, solar energy is utilized to carry out the
chemical reaction. Therefore, heterogeneous and homogeneous photocatalytic í µí°¶í µí± 2
reduction could be a promising way to tackle the dual problem of global warming
and meeting the increasing energy demand.
4 Advantages of Photocatalytic Conversion Process over
Thermocatalytic Conversion
The CO 2 conversion process takes place at moderate reaction conditions, i.e., at
atmospheric pressure and temperature, unlike in case of thermocatalytic process
where high temperature and pressures are required. Energy source in case of photocatalytic process is solar light which is present in enormous amount. In photocatalytic route, the hydrogen source is water, while in the case of
thermocatalytic conversion process, the source of hydrogen is H 2 gas. Hence, cost
of water as a hydrogen source in photocatalytic process is lesser compared to
hydrogen gas. The total energy input cost in case of photocatalytic is far less compared to that in case of thermocatalytic conversion process. The photocatalysts
used in case of photoconversion processes possess the quality of recyclability. As
the conversion process takes place at low temperature and pressure in case of photocatalytic conversion, the number of cycles for which catalysts can be used are
more in case of photocatalyst when compared to the recyclability of the thermocatalyst. With lesser initial investment and higher duration of use, the photocatalytic process is a promising means to utilize CO 2 for conversion to value-added
chemicals.
The photocatalysts are majorly semiconductors, possessing band structure with
conduction and valance bands. The highest occupied energy band that is completely
occupied is the valance band, and the energy band from top that is unoccupied is the
conduction band. The difference between the valance and conduction band is the
band gap, and through thermal interactions, the electrons can move from valance to
conduction band and vice versa. Conduction band energy is higher compared to that
of the valance band. The population density of the charge carriers, i.e., electron
(negative charge carrier) and holes (positive charge carriers), can change upon interaction with light. For the activation of stable CO 2 molecules, the energy input is
desired; in photocatalytic process, this energy is provided by the high-energy electrons. The high-energy electrons are the electrons that are present in the conduction
K. Tripathi et al.