171
6.3 The Design and Fabrication of Catalysts for Carbon
Dioxide Photoreduction to Formaldehyde
There are many tasks that should be done to accomplish a great efficiency and selectivity of the photocatalytic CO 2 reduction, including (i) enhancing light harvesting,
(ii) promoting separation of the photoproduced charges, (iii) improving CO 2 adsorption and activation, (iv) accelerating kinetics of CO 2 reduction, and (v) suppressing
undesired processes. For the first two tasks, considerable improvements have been
realized because these tasks are similar to the tasks as the photocatalytic watersplitting process, which has been extensively studied. While the next two tasks are
practically feasible as discussed before, the last one is still the most challenging
issue because of the intense competition with H 2 evolution reaction (HER) in H 2 O
media, which is more abundant and easily adsorbed on the catalyst surfaces comparing to CO 2 .
6.3.1 Increased Visible-Light Excitation
Strengthening the excitation flow of photo-generated electron-hole pairs is mostly
derived from decreasing the E g of semiconductors via harvesting visible-light
region. De facto, these visible-light-driven photocatalytic systems acting under
more spreading visible-light absorption are more likely to provide a varied range of
applications owing to their photoactivity within the ultraviolet region only. As a
result, a number of works have attempted to boost the possibility of visible-lightdriven excitation by the physical/chemical modification of available photocatalysts.
Designing such novel photocatalytic systems for CO 2 reduction to HCHO can be
divided into five main approaches including (i) impurity doping, (ii) forming structural defectiveness, (iii) sensitization, (iv) surface plasmon resonance (SPR) effect,
and (v) solid solution.
Firstly, impurity doping is a modification process that attaches the exotic
component(s) to an inherent structure, aiming at varying the electronic essence and
visible-light response ability of a nanostructured semiconductor. This strategy
might bring two primary advantages as follows: (1) reaching the two-stage excitation states and shortening the overall bandgap energies and (2) shifting the ultraviolet excitation region (λ = 10–400 nm) to visible-light region (λ > 400 nm) of
nonmetal particle-doped nanocomposites. It is possible that the nitrogen doping
along with the mesoporous structure of TiO 2 heterogeneous catalyst, for example,
suppressed the process of recombined e
−
and h
+
, thus resulting in an enhancement of
photoactivity (Li et al. 2012). For example, Chai and Guo (2016) synthesized the
N-doped graphene/carbon nanotube catalysts for CO 2 electrochemical reduction
with highly selective HCHO and CH 3 OH formation. Meanwhile, noble metal-doped
photocatalysts may generate a certain impurity level and also elevate their absorbability towards visible light. Electron traps and active sites are two main reasons that
6 Conversion of Carbon Dioxide into Formaldehyde
6.3 The Design and Fabrication of Catalysts for Carbon
Dioxide Photoreduction to Formaldehyde
There are many tasks that should be done to accomplish a great efficiency and selectivity of the photocatalytic CO 2 reduction, including (i) enhancing light harvesting,
(ii) promoting separation of the photoproduced charges, (iii) improving CO 2 adsorption and activation, (iv) accelerating kinetics of CO 2 reduction, and (v) suppressing
undesired processes. For the first two tasks, considerable improvements have been
realized because these tasks are similar to the tasks as the photocatalytic watersplitting process, which has been extensively studied. While the next two tasks are
practically feasible as discussed before, the last one is still the most challenging
issue because of the intense competition with H 2 evolution reaction (HER) in H 2 O
media, which is more abundant and easily adsorbed on the catalyst surfaces comparing to CO 2 .
6.3.1 Increased Visible-Light Excitation
Strengthening the excitation flow of photo-generated electron-hole pairs is mostly
derived from decreasing the E g of semiconductors via harvesting visible-light
region. De facto, these visible-light-driven photocatalytic systems acting under
more spreading visible-light absorption are more likely to provide a varied range of
applications owing to their photoactivity within the ultraviolet region only. As a
result, a number of works have attempted to boost the possibility of visible-lightdriven excitation by the physical/chemical modification of available photocatalysts.
Designing such novel photocatalytic systems for CO 2 reduction to HCHO can be
divided into five main approaches including (i) impurity doping, (ii) forming structural defectiveness, (iii) sensitization, (iv) surface plasmon resonance (SPR) effect,
and (v) solid solution.
Firstly, impurity doping is a modification process that attaches the exotic
component(s) to an inherent structure, aiming at varying the electronic essence and
visible-light response ability of a nanostructured semiconductor. This strategy
might bring two primary advantages as follows: (1) reaching the two-stage excitation states and shortening the overall bandgap energies and (2) shifting the ultraviolet excitation region (λ = 10–400 nm) to visible-light region (λ > 400 nm) of
nonmetal particle-doped nanocomposites. It is possible that the nitrogen doping
along with the mesoporous structure of TiO 2 heterogeneous catalyst, for example,
suppressed the process of recombined e
−
and h
+
, thus resulting in an enhancement of
photoactivity (Li et al. 2012). For example, Chai and Guo (2016) synthesized the
N-doped graphene/carbon nanotube catalysts for CO 2 electrochemical reduction
with highly selective HCHO and CH 3 OH formation. Meanwhile, noble metal-doped
photocatalysts may generate a certain impurity level and also elevate their absorbability towards visible light. Electron traps and active sites are two main reasons that
6 Conversion of Carbon Dioxide into Formaldehyde