12.4.2 Improve CO 2 Adsorption and Activation
The CO 2 adsorption and activation on the surface of photocatalyst are two important
steps; however, without modification, common semiconductor-based photocatalysts
often show low CO 2 uptake. Therefore, combining some unique cocatalysts with
higher CO 2 adsorption amount should be a proper way to improve CO2PR efficiency. Xie et al. [66] in the use of MgO, a basic metal oxide, as the cocatalyst
deposited on the TiO 2 surface, with the addition of Pt NPs, the Pt-MgO/TiO 2
composite shows an enhanced activity for CH 4 production. During the experiments,
a linear relationship between different CO 2 chemisorption by different basic metal
oxide-modified Pt–TiO 2 and CH 4 yields clearly demonstrates the important role of
CO 2 adsorption; the MgO modification shows the highest CO 2 chemisorption
compared with other basic metal oxides. Besides, the optimal MgO content is
measured to be 1%; excess MgO adding will cause a thicker MgO layer and cover
the Pt sites which are detrimental for CO 2 photoreduction. Li et al. [53] adopt MOF
(Cu 3 (BTC) 2 ) as the CO 2 adsorption cocatalyst and coat porous TiO 2 shell on the
MOF crystals’ surface. This unique design hybrid shows enhanced CH 4 yield and
selectivity compared with bare TiO 2 counterpart. The CO 2 adsorption results
between bare MOF and MOF@TiO 2 suggest the CO 2 molecules can easily pass
the TiO 2 shell. In order to investigate the charge transfer and working mechanism,
the author adopts TA analysis and first-principle simulation. The result indicates the
photo-excited electrons can transfer to the MOF core; subsequently, the CO 2 molecules adsorbed in MOF can be activated and convert into CH 4 effectively. Similarly, Shi et al. [56] reported a C 3 N 4 /UiO-66 composite, in this work zirconiumbased MOF: UiO-66 acts as both CO 2 absorber and a semiconductor-like material to
promote the electron-hole separation. ESR was used to verify the electron transfer
route. Specifically, signal of g ¼ 2.009 is attribute to O 2
À which was found in C 3 N 4 /
UiO-66 under visible light irradiation but absent in pristine UiO-66. This indicates
that the C 3 N 4 was performed as a photosensitizer; the photo-generated electrons
transferred to UiO-66 and thus suppress the electron-hole recombination and
enhance the CO2PR performance. Pan et al. [47] reported a carbon-coated In 2 O 3
photocatalyst with the use of glucose as the carbon source; the 5 nm carbon layer
could enhance the CO 2 chemisorption and suppress the hydrogen generation
(Fig. 12.9). Compared with the pure In 2 O 3 nanobelt, C-In 2 O 3 shows enhanced
CO 2 adsorption capacity compared with pristine In 2 O 3 nanobelt, and the maximum
CO 2 adsorption was reached with the use of 0.8 g glucose (Fig. 12.9b). The
selectivity of CH 4 was studied by the thermodynamic and kinetic behavior of H
proton transfer route in the assistance of DFT calculation. The result indicates that
the H proton transfer to adsorbed CO 2 in Pt 2 /C-In 2 O 3 is easier than H 2 formation
(endothermic); on the contrary, H proton reduction to H 2 on Pt 2 /P-In 2 O 3 is exothermic, which is easier than Pt 2 /C-In 2 O 3. This result well-explained the high CH 4 yield
and CO2PR selectivity of Pt/C-In 2 O 3 compared with Pt/P-In 2 O 3 .
12.4 Roles and Properties of Different Cocatalysts
297
The CO 2 adsorption and activation on the surface of photocatalyst are two important
steps; however, without modification, common semiconductor-based photocatalysts
often show low CO 2 uptake. Therefore, combining some unique cocatalysts with
higher CO 2 adsorption amount should be a proper way to improve CO2PR efficiency. Xie et al. [66] in the use of MgO, a basic metal oxide, as the cocatalyst
deposited on the TiO 2 surface, with the addition of Pt NPs, the Pt-MgO/TiO 2
composite shows an enhanced activity for CH 4 production. During the experiments,
a linear relationship between different CO 2 chemisorption by different basic metal
oxide-modified Pt–TiO 2 and CH 4 yields clearly demonstrates the important role of
CO 2 adsorption; the MgO modification shows the highest CO 2 chemisorption
compared with other basic metal oxides. Besides, the optimal MgO content is
measured to be 1%; excess MgO adding will cause a thicker MgO layer and cover
the Pt sites which are detrimental for CO 2 photoreduction. Li et al. [53] adopt MOF
(Cu 3 (BTC) 2 ) as the CO 2 adsorption cocatalyst and coat porous TiO 2 shell on the
MOF crystals’ surface. This unique design hybrid shows enhanced CH 4 yield and
selectivity compared with bare TiO 2 counterpart. The CO 2 adsorption results
between bare MOF and MOF@TiO 2 suggest the CO 2 molecules can easily pass
the TiO 2 shell. In order to investigate the charge transfer and working mechanism,
the author adopts TA analysis and first-principle simulation. The result indicates the
photo-excited electrons can transfer to the MOF core; subsequently, the CO 2 molecules adsorbed in MOF can be activated and convert into CH 4 effectively. Similarly, Shi et al. [56] reported a C 3 N 4 /UiO-66 composite, in this work zirconiumbased MOF: UiO-66 acts as both CO 2 absorber and a semiconductor-like material to
promote the electron-hole separation. ESR was used to verify the electron transfer
route. Specifically, signal of g ¼ 2.009 is attribute to O 2
À which was found in C 3 N 4 /
UiO-66 under visible light irradiation but absent in pristine UiO-66. This indicates
that the C 3 N 4 was performed as a photosensitizer; the photo-generated electrons
transferred to UiO-66 and thus suppress the electron-hole recombination and
enhance the CO2PR performance. Pan et al. [47] reported a carbon-coated In 2 O 3
photocatalyst with the use of glucose as the carbon source; the 5 nm carbon layer
could enhance the CO 2 chemisorption and suppress the hydrogen generation
(Fig. 12.9). Compared with the pure In 2 O 3 nanobelt, C-In 2 O 3 shows enhanced
CO 2 adsorption capacity compared with pristine In 2 O 3 nanobelt, and the maximum
CO 2 adsorption was reached with the use of 0.8 g glucose (Fig. 12.9b). The
selectivity of CH 4 was studied by the thermodynamic and kinetic behavior of H
proton transfer route in the assistance of DFT calculation. The result indicates that
the H proton transfer to adsorbed CO 2 in Pt 2 /C-In 2 O 3 is easier than H 2 formation
(endothermic); on the contrary, H proton reduction to H 2 on Pt 2 /P-In 2 O 3 is exothermic, which is easier than Pt 2 /C-In 2 O 3. This result well-explained the high CH 4 yield
and CO2PR selectivity of Pt/C-In 2 O 3 compared with Pt/P-In 2 O 3 .
12.4 Roles and Properties of Different Cocatalysts
297