166
alkali solutions, CO 2 is well dissolved as bicarbonate and/or carbonate species in the
reaction medium, but consequently, its reduction potential is increased, leading to
the decreasing efficiency of the reduction process. Although using methanol or isopropanol as a solvent has been reported to be practically feasible to enhance CO 2
solubility and resulting photoactivity (Sasirekha et al. 2006; Peng et al. 2012; Chan
et al. 2018), it makes the process economically and environmentally unfavorable.
Therefore, boosting the reaction conditions, such as increasing pressure to enhance
CO 2 solubility, seems to be the most feasible strategy that has been investigated so far.
6.2.2 The Overall Photocatalytic Carbon Dioxide Reduction
Process
Figure 6.3 shows the process for photocatalytic CO 2 reduction to HCHO and other
products as CH 4 , CO, or CH 3 OH, including eight steps such as excitation, transportation, separation, electrocatalytic reduction of CO 2 , and water oxidation (Li et al.
2014a; Wu et al. 2017; Shehzad et al. 2018).
Assuming there is a semiconductor material that has bandgap energy E g as soon
as its surface absorbs photons that have equal or greater energy than E g , its electrons
(e
−
) in the valence band will be jumped to the conduction band and leave the holes
(h
+
) there (step 1 of Fig. 6.3). Those photoexcited electron-hole pairs will then possibly participate in the following steps. In step 2, the photoexcited electron-hole
pairs are separated and migrated to the semiconductor surface. In step 4 and step 5,
the electrocatalytic CO 2 reduction by photoexcited electrons occurs, and the oxida-3
-2
-1
0
+1
+2
+3
(pH = 7)
TiO 2 (R) TiO 2 (A)
BiVO 4
CdSe
ZnO
Potential vs. NHE (V)
SrTiO 3
Si
TaON
CdS
GaP
SiC
ZnS
CO 2 /HCOOH (-0.61 V)
CO 2 /HCHO (-0.48 V)
CO 2 /CH 4 OH (-0.38 V)
CO 2 /CH 4 (-0.24 V)
H 2 O/O 2 (0.82 V)
2H
+ /H 2 (-0.41 V)
Cu 2 O
C 3 N 4
Ta 3 N 5
Fig. 6.2 Comparison of the standard electrochemical potentials for reduction of CO 2 at neutral
condition and band positions of referenced semiconductors. (Adapted from Li et al. (2014b) with
copyright permission 2014, Springer Nature. (NHE Normal Hydrogen Electrode))
T. D. Nguyen et al.
alkali solutions, CO 2 is well dissolved as bicarbonate and/or carbonate species in the
reaction medium, but consequently, its reduction potential is increased, leading to
the decreasing efficiency of the reduction process. Although using methanol or isopropanol as a solvent has been reported to be practically feasible to enhance CO 2
solubility and resulting photoactivity (Sasirekha et al. 2006; Peng et al. 2012; Chan
et al. 2018), it makes the process economically and environmentally unfavorable.
Therefore, boosting the reaction conditions, such as increasing pressure to enhance
CO 2 solubility, seems to be the most feasible strategy that has been investigated so far.
6.2.2 The Overall Photocatalytic Carbon Dioxide Reduction
Process
Figure 6.3 shows the process for photocatalytic CO 2 reduction to HCHO and other
products as CH 4 , CO, or CH 3 OH, including eight steps such as excitation, transportation, separation, electrocatalytic reduction of CO 2 , and water oxidation (Li et al.
2014a; Wu et al. 2017; Shehzad et al. 2018).
Assuming there is a semiconductor material that has bandgap energy E g as soon
as its surface absorbs photons that have equal or greater energy than E g , its electrons
(e
−
) in the valence band will be jumped to the conduction band and leave the holes
(h
+
) there (step 1 of Fig. 6.3). Those photoexcited electron-hole pairs will then possibly participate in the following steps. In step 2, the photoexcited electron-hole
pairs are separated and migrated to the semiconductor surface. In step 4 and step 5,
the electrocatalytic CO 2 reduction by photoexcited electrons occurs, and the oxida-3
-2
-1
0
+1
+2
+3
(pH = 7)
TiO 2 (R) TiO 2 (A)
BiVO 4
CdSe
ZnO
Potential vs. NHE (V)
SrTiO 3
Si
TaON
CdS
GaP
SiC
ZnS
CO 2 /HCOOH (-0.61 V)
CO 2 /HCHO (-0.48 V)
CO 2 /CH 4 OH (-0.38 V)
CO 2 /CH 4 (-0.24 V)
H 2 O/O 2 (0.82 V)
2H
+ /H 2 (-0.41 V)
Cu 2 O
C 3 N 4
Ta 3 N 5
Fig. 6.2 Comparison of the standard electrochemical potentials for reduction of CO 2 at neutral
condition and band positions of referenced semiconductors. (Adapted from Li et al. (2014b) with
copyright permission 2014, Springer Nature. (NHE Normal Hydrogen Electrode))
T. D. Nguyen et al.