176
catalysts have been prepared, and they displayed the efficient photocatalytic activity
for CO 2 photoreduction to fuels. For example, Ti-based MCM-48 mesoporous zeolite was used as an efficient catalyst for the production of CH 4 , CH 3 OH, and HCHO
mixture via CO 2 reduction (Anpo et al. 1998).
To accelerate the CO 2 photoreduction to formaldehyde, the development of transition metal-based co-catalyzed loading CO 2 reduction has been discussed.
Theoretically, the main role of co-catalysts relies on changing the electrochemical
potential, engendering the electron-hole separation and charge transport, or allowing to reach a wide range of bandgap energy under visible-light-driven conditions.
Moreover, co-catalysts-modified systems can provide more electron traps to suppress the recombination e
−
and h
+
pairs, resulting in high selectivity, performance,
and efficiency for CO 2 reduction. For example, silver co-catalyzed Ca/Sr/
BaLa 4 Ti 4 O 15 revealed higher photoactivity for CO 2 reduction to CO and HCOOH
(Iizuka et al. 2011). Other bimetallic catalytic systems showed the same performance for CO 2 reduction rather than single metallic co-catalysts (Bontemps
et al. 2014).
Improving water oxidation kinetics has also been discussed to accelerate the CO 2
photoreduction to formaldehyde. In normal, a major problem that is detrimental for
the CO 2 photoreduction is in situ oxidized products or recombination, resulting in a
decrease in photocatalytic performance. Thus, to overcome this challenge, electrondonor sacrificial agents such as ethylenediaminetetraacetic acid, sodium sulfide,
acetonitrile, alcohols, and amines can be used. Especially, it is of significance to
consider the amounts of O 2 from the CO 2 photoreduction in H 2 O media. A good
example is that tertiary amines (R 3 N) could be added to recycle electron donor,
helping for CO 2 photoreduction combined with water splitting to enhance overall
performance yield (Richardson et al. 2011). Therefore, CO 2 photoreduction along
with H 2 O splitting could be innovative ways to solve mentioned problems.
6.3.5 Minimizing Undesirable Reactions
Generally, there are several pathways to inhibit undesirable reactions, thus reaching
cost-effectiveness for the overall photocatalytic process. Among these common
routes, two main strategies including inhibiting hydrogen evolution and products
oxidation are primarily focused here.
Firstly, hydrogen evolution is the side reaction, which competes with CO 2 photoreduction, causing a considerable depletion in the efficiency and selectivity for CO 2
photoreduction. However, the kinetics of this reaction may become more favorable
when it is coupled with the secondary process such as water splitting. Typically,
Zhai et al. (2013) used the Pt@Cu 2 O co-catalyst on TiO 2 template as a robust photocatalytic system to convert CO 2 with H 2 O to fuels with up to 85% of selectivity
and without generating H 2 under the reduction of water. Cu 2 O core-shell was attributable to the provision of active sites for CO 2 activation, while Pt sites might separate the photo-generated electrons from TiO 2 , contributing to CO 2 transformation.
T. D. Nguyen et al.
catalysts have been prepared, and they displayed the efficient photocatalytic activity
for CO 2 photoreduction to fuels. For example, Ti-based MCM-48 mesoporous zeolite was used as an efficient catalyst for the production of CH 4 , CH 3 OH, and HCHO
mixture via CO 2 reduction (Anpo et al. 1998).
To accelerate the CO 2 photoreduction to formaldehyde, the development of transition metal-based co-catalyzed loading CO 2 reduction has been discussed.
Theoretically, the main role of co-catalysts relies on changing the electrochemical
potential, engendering the electron-hole separation and charge transport, or allowing to reach a wide range of bandgap energy under visible-light-driven conditions.
Moreover, co-catalysts-modified systems can provide more electron traps to suppress the recombination e
−
and h
+
pairs, resulting in high selectivity, performance,
and efficiency for CO 2 reduction. For example, silver co-catalyzed Ca/Sr/
BaLa 4 Ti 4 O 15 revealed higher photoactivity for CO 2 reduction to CO and HCOOH
(Iizuka et al. 2011). Other bimetallic catalytic systems showed the same performance for CO 2 reduction rather than single metallic co-catalysts (Bontemps
et al. 2014).
Improving water oxidation kinetics has also been discussed to accelerate the CO 2
photoreduction to formaldehyde. In normal, a major problem that is detrimental for
the CO 2 photoreduction is in situ oxidized products or recombination, resulting in a
decrease in photocatalytic performance. Thus, to overcome this challenge, electrondonor sacrificial agents such as ethylenediaminetetraacetic acid, sodium sulfide,
acetonitrile, alcohols, and amines can be used. Especially, it is of significance to
consider the amounts of O 2 from the CO 2 photoreduction in H 2 O media. A good
example is that tertiary amines (R 3 N) could be added to recycle electron donor,
helping for CO 2 photoreduction combined with water splitting to enhance overall
performance yield (Richardson et al. 2011). Therefore, CO 2 photoreduction along
with H 2 O splitting could be innovative ways to solve mentioned problems.
6.3.5 Minimizing Undesirable Reactions
Generally, there are several pathways to inhibit undesirable reactions, thus reaching
cost-effectiveness for the overall photocatalytic process. Among these common
routes, two main strategies including inhibiting hydrogen evolution and products
oxidation are primarily focused here.
Firstly, hydrogen evolution is the side reaction, which competes with CO 2 photoreduction, causing a considerable depletion in the efficiency and selectivity for CO 2
photoreduction. However, the kinetics of this reaction may become more favorable
when it is coupled with the secondary process such as water splitting. Typically,
Zhai et al. (2013) used the Pt@Cu 2 O co-catalyst on TiO 2 template as a robust photocatalytic system to convert CO 2 with H 2 O to fuels with up to 85% of selectivity
and without generating H 2 under the reduction of water. Cu 2 O core-shell was attributable to the provision of active sites for CO 2 activation, while Pt sites might separate the photo-generated electrons from TiO 2 , contributing to CO 2 transformation.
T. D. Nguyen et al.