168
Apparently, steps 7 and 8 are unfavorable for photocatalytic CO 2 reduction. The
former step represents for the electrocatalytic H 2 evolution, which occurs by trapping photoexcited electrons in CRC, causing a significant decrease in the available
electrons for reduction of CO 2 . Meanwhile, the later one represents the electrocatalytic oxidation of the reduction products by WOC, resulting in a negative effect on
the oxidation of water and CO 2 reduction. Thus, these two unfavorable processes
would remarkably decrease the quantum yield of semiconductors for the photocatalytic CO 2 reduction. In short, these factors should be thoroughly considered in the
fabrication and design of greatly efficient photocatalysts to avoid or diminish the
abovementioned unfavorable processes for effective photocatalytic CO 2 reduction.
6.2.3 Kinetics and Mechanism Aspects
The mechanistic pathways for photocatalytic CO 2 reduction in liquid phase have
been examined and published in literature. Nevertheless, these mechanistic steps
have not been fully elucidated. In this section, two mechanisms are comprehensively discussed to verify the intrinsic pathway for photocatalytic reduction of CO 2
as well as products formation.
Firstly, Fig. 6.4 describes the possible mechanism for C 1 -product formation
including HCOOH, HCHO, and CH 3 OH from CO 2 over photocatalyst, which was
developed by Qin et al. (2013). They demonstrated that photocatalytic reduction of
CO 2 is a complicated redox process, which involves the oxidation of reductant and
a multistep reduction process of CO 2 (Eq. (6.1)) with being controlled by the reaction media; thus, the mechanisms in acidic solution and alkaline solution were proposed. In acidic medium, it starts with the absorption of CO 2 on the surface of
photocatalyst to generate A—a distorted molecular, resulting in the stretch of the
C=O bond and activation of its π–bond. Then, the reaction of the activated C=O
bond with an e
−
generates B, C
●
–O
−
species. Besides, H 2 O, which acts as a reductant, reacts with h
+
to form OH
−
and H
+
, and then H
+
receives e
−
for forming H
●
. The
reaction between CO 2
●–
intermediate and H
●
could yield C, HCOO. If the reaction
is taken place in an acidic environment, then H
+
ions can interact with HCOO
−
species to form D, absorbed HCOOH. It is then desorbed from the catalyst surface to
form E, thereby releasing vacant sites for absorbing other CO 2 species. D can be
participated in further reduction reactions to form HCHO or CH 3 OH. In the case,
the reaction occurs under alkaline conditions, HCO 3
−
and CO 3
2−
species are formed
from bubbling CO 2 , and their concentrations increase with increasing pH value. C,
HCOO
−
interacts with HCO 3
−
ion to desorb C from the surface of photocatalyst,
giving G. The yield of G is lower than that of D due to the more difficult conversion
of the intermediate F to G in alkaline conditions. The absorbed C tends to attack
another e
−
to form H and then reacts with HCO 3
−
and •H to form intermediate
J. HCHO can be formed by eliminating H 2 O from J and further reduced to form
CH 3 OH. Since C and HCHO cannot be desorbed effortlessly from the photocatalyst
surface under alkaline conditions, they are observed to be unendingly reduced to
form CH 3 OH at pH 10.
T. D. Nguyen et al.
Apparently, steps 7 and 8 are unfavorable for photocatalytic CO 2 reduction. The
former step represents for the electrocatalytic H 2 evolution, which occurs by trapping photoexcited electrons in CRC, causing a significant decrease in the available
electrons for reduction of CO 2 . Meanwhile, the later one represents the electrocatalytic oxidation of the reduction products by WOC, resulting in a negative effect on
the oxidation of water and CO 2 reduction. Thus, these two unfavorable processes
would remarkably decrease the quantum yield of semiconductors for the photocatalytic CO 2 reduction. In short, these factors should be thoroughly considered in the
fabrication and design of greatly efficient photocatalysts to avoid or diminish the
abovementioned unfavorable processes for effective photocatalytic CO 2 reduction.
6.2.3 Kinetics and Mechanism Aspects
The mechanistic pathways for photocatalytic CO 2 reduction in liquid phase have
been examined and published in literature. Nevertheless, these mechanistic steps
have not been fully elucidated. In this section, two mechanisms are comprehensively discussed to verify the intrinsic pathway for photocatalytic reduction of CO 2
as well as products formation.
Firstly, Fig. 6.4 describes the possible mechanism for C 1 -product formation
including HCOOH, HCHO, and CH 3 OH from CO 2 over photocatalyst, which was
developed by Qin et al. (2013). They demonstrated that photocatalytic reduction of
CO 2 is a complicated redox process, which involves the oxidation of reductant and
a multistep reduction process of CO 2 (Eq. (6.1)) with being controlled by the reaction media; thus, the mechanisms in acidic solution and alkaline solution were proposed. In acidic medium, it starts with the absorption of CO 2 on the surface of
photocatalyst to generate A—a distorted molecular, resulting in the stretch of the
C=O bond and activation of its π–bond. Then, the reaction of the activated C=O
bond with an e
−
generates B, C
●
–O
−
species. Besides, H 2 O, which acts as a reductant, reacts with h
+
to form OH
−
and H
+
, and then H
+
receives e
−
for forming H
●
. The
reaction between CO 2
●–
intermediate and H
●
could yield C, HCOO. If the reaction
is taken place in an acidic environment, then H
+
ions can interact with HCOO
−
species to form D, absorbed HCOOH. It is then desorbed from the catalyst surface to
form E, thereby releasing vacant sites for absorbing other CO 2 species. D can be
participated in further reduction reactions to form HCHO or CH 3 OH. In the case,
the reaction occurs under alkaline conditions, HCO 3
−
and CO 3
2−
species are formed
from bubbling CO 2 , and their concentrations increase with increasing pH value. C,
HCOO
−
interacts with HCO 3
−
ion to desorb C from the surface of photocatalyst,
giving G. The yield of G is lower than that of D due to the more difficult conversion
of the intermediate F to G in alkaline conditions. The absorbed C tends to attack
another e
−
to form H and then reacts with HCO 3
−
and •H to form intermediate
J. HCHO can be formed by eliminating H 2 O from J and further reduced to form
CH 3 OH. Since C and HCHO cannot be desorbed effortlessly from the photocatalyst
surface under alkaline conditions, they are observed to be unendingly reduced to
form CH 3 OH at pH 10.
T. D. Nguyen et al.