175
acidic compound, along with electron-rich oxygen atoms, showing the H-acceptors
in the presence of H-donors. Taking advantage of these properties, basic functional
groups such as amines, hydroxides, and H-donors can be attached on photocatalyst
surface to accrue the CO 2 adsorption. For example, the amine-functionalized of
TiO 2 nanoparticles intensified the chemisorption towards CO 2 , thus promoting C–O
bond activation (Liao et al. 2014). More specific, the combination of amine functionalization and highly porosity on the C 3 N 4 photocatalysts also bring many outstanding advantages.
Finally, to enhance adsorption and activation of CO 2 , focusing on factors affecting CO 2 activation is also investigated. Apart from the role of oxygen vacancies as
mentioned above, a series of other factors can affect the activation of C=O bonds.
For example, the polarity and relative permittivity of solvents and the hydrophobic–
hydrophilic interactions on the photocatalysts surface have profound impacts on the
activation of CO 2 and the stability of CO
−
, resulting in the efficiency and selectivity
of the photocatalytic CO 2 reduction (Liu et al. 1997). Therefore, the selection of
appropriate solvents, carriers, and catalysts is further studied to enhance the transformation of CO 2 into HCHO.
6.3.4 Accelerated Carbon Dioxide Reduction Kinetics
To accelerate the CO 2 photoreduction to formaldehyde, the materials can be synthesized whether upon a porosity, functionalization, and light harvesting enhancement
for mesoporous structures to improve the accessible ability of CO 2 towards photocatalytic sites or transition metal-based co-catalyzed loading CO 2 reduction to facilitate e
−
and h
+
separation, charge transport, or water oxidation kinetic improvement.
Every circumstance includes the specific advantages and disadvantages and thus
needs to be discussed in detail.
Synthesizing a new generation of mesoporous photocatalytic materials is also
considered. It was reported that mesoporous gallium-based photocatalyst showed
many times the higher surface area, hence leading to stronger photoactivity towards
CO 2 than those of solid-state reaction-synthesized counterpart sample (Zhang et al.
2012). Similarly, mesoporous In(OH) 3 material exhibited so far better efficiency for
CO 2 photoreduction to fuels compared with non-mesoporous samples (Xu et al.
2013), while Park and partners described a new photocatalytic generation of mesoporous Ga 2 O 3 nanoparticles with the considerable enhancement in the reduction of
CO 2 in comparison to bulk nanoparticles (Park et al. 1901). Interestingly, the specific surface area and crystallinity of mesoporous graphitic C 3 N 4 had a significant
effect on the CO 2 photoreduction to HCHO and HCOOH compounds, while the
pore size and volume effects were negligible (Park et al. 1901). However, these
kinds of photocatalysts are thermally and chemically unstable under harsh conditions such as high temperature and highly acidic media. Among the stable nanoscaled
materials, mesoporous zeolites present the high thermal properties along with open
metal sites and high surface area. In fact, zeolites-supported first-row-metals photo6 Conversion of Carbon Dioxide into Formaldehyde
acidic compound, along with electron-rich oxygen atoms, showing the H-acceptors
in the presence of H-donors. Taking advantage of these properties, basic functional
groups such as amines, hydroxides, and H-donors can be attached on photocatalyst
surface to accrue the CO 2 adsorption. For example, the amine-functionalized of
TiO 2 nanoparticles intensified the chemisorption towards CO 2 , thus promoting C–O
bond activation (Liao et al. 2014). More specific, the combination of amine functionalization and highly porosity on the C 3 N 4 photocatalysts also bring many outstanding advantages.
Finally, to enhance adsorption and activation of CO 2 , focusing on factors affecting CO 2 activation is also investigated. Apart from the role of oxygen vacancies as
mentioned above, a series of other factors can affect the activation of C=O bonds.
For example, the polarity and relative permittivity of solvents and the hydrophobic–
hydrophilic interactions on the photocatalysts surface have profound impacts on the
activation of CO 2 and the stability of CO
−
, resulting in the efficiency and selectivity
of the photocatalytic CO 2 reduction (Liu et al. 1997). Therefore, the selection of
appropriate solvents, carriers, and catalysts is further studied to enhance the transformation of CO 2 into HCHO.
6.3.4 Accelerated Carbon Dioxide Reduction Kinetics
To accelerate the CO 2 photoreduction to formaldehyde, the materials can be synthesized whether upon a porosity, functionalization, and light harvesting enhancement
for mesoporous structures to improve the accessible ability of CO 2 towards photocatalytic sites or transition metal-based co-catalyzed loading CO 2 reduction to facilitate e
−
and h
+
separation, charge transport, or water oxidation kinetic improvement.
Every circumstance includes the specific advantages and disadvantages and thus
needs to be discussed in detail.
Synthesizing a new generation of mesoporous photocatalytic materials is also
considered. It was reported that mesoporous gallium-based photocatalyst showed
many times the higher surface area, hence leading to stronger photoactivity towards
CO 2 than those of solid-state reaction-synthesized counterpart sample (Zhang et al.
2012). Similarly, mesoporous In(OH) 3 material exhibited so far better efficiency for
CO 2 photoreduction to fuels compared with non-mesoporous samples (Xu et al.
2013), while Park and partners described a new photocatalytic generation of mesoporous Ga 2 O 3 nanoparticles with the considerable enhancement in the reduction of
CO 2 in comparison to bulk nanoparticles (Park et al. 1901). Interestingly, the specific surface area and crystallinity of mesoporous graphitic C 3 N 4 had a significant
effect on the CO 2 photoreduction to HCHO and HCOOH compounds, while the
pore size and volume effects were negligible (Park et al. 1901). However, these
kinds of photocatalysts are thermally and chemically unstable under harsh conditions such as high temperature and highly acidic media. Among the stable nanoscaled
materials, mesoporous zeolites present the high thermal properties along with open
metal sites and high surface area. In fact, zeolites-supported first-row-metals photo6 Conversion of Carbon Dioxide into Formaldehyde