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Catalysts 2020, 10, 1287 5 of 25 electrons in the CB. Commonly, photocatalytic CO2 reduction is performed in fluidized bed reactor or optical fiber reactor [27]. In fluidized bed reactor [30], the photocatalysts are well dispersed in aqueous solution, thus promoting the contact and reaction between the photocatalysts and water soluble CO2, but it suffers from the hard separation of products and low light utilization efficiency. By contrast, the photocatalysts coating on the optical fibers in optical fiber reactor [31], considerably improve the illuminated surface area of photocatalysts and light utilization efficiency. Thus, it may be a promising technique to enhance photocatalytic CO2 reduction efficiency. The CO2 photoreduction process generally undergo four major steps: (1) CO2 molecules are chemically adsorbed on the surface of photocatalysts; (2) under light illumination, the electrons of semiconductor photocatalysts can be excited by photons from VB to CB, leaving an equal number of holes in the VB; (3) the photogenerated electrons are separated from holes and migrate to the photocatalyst surface; (4) the electrons are used to activate and reduce CO2 into solar fuels, while the holes are consumed by the oxidation of H2O [32]. According to reactions in Table 1, the photocatalytic CO2 reduction products are different over various photocatalysts with different CB and VB positions, which is related to the number of electrons and protons (e− /H+ ) involved in reduction reactions. Actually, one electron involved reaction in the reduction of CO2 is highly unfavorable thermodynamically due totheverynegativeredoxpotentialofCO +e−=CO−(−1.90Vvs.NHE)[22].Therefore,multiple 22 electrons and a corresponding number of protons must be involved in the photocatalytic CO2 reduction reactions. The clean fuel ethanol can be produced from the CO2 photoreduction reaction involving twelve electrons and twelve protons, which requires a suitable photocatalyst with multiple electrons easily migrating from a photocatalyst to CO2. From the point of view of the four photocatalytic steps, a highly active photocatalyst should possess the following characteristics: (1) a large surface area for increasing the adsorption of CO2 and the surface active sites; (2) a narrow bandgap and proper band positions for utilizing solar energy effectively; (3) a nanostructure favorable for electron transport and improving the separation of photogenerated electron–hole pairs; (4) abundant surface oxygen vacancies for changing the electronic and chemical properties of the semiconductor surfaces and facilitating CO2 adsorption/activation. Additionally, the co-catalysts are usually attached on the surface of photocatalysts to promote the separation and migration of photo-induced carriers, and effectively lower the reaction energy barrier for CO2 activation and reduction [33]. 2.3. CO2 Photoelectroreduction Photoelectrocatalytic reduction of CO2 is considered as an integration of photocatalytic and electrocatalytic CO2 reduction, where the solar energy and electricity synergistically promotes the conversion of CO2 to clean fuels. During the CO2 photoelectroreduction process, the applied potential facilitates the separation of photogenerated electron–hole pairs in the photocatalytic step, and, in turn, the extra light irradiation could reduce the overpotential in the electrcatalytic step [14]. Photoelectrocatalytic CO2 reduction system employs semiconductor materials as the photocathodes that can not only used as catalysts, but also as the light harvesting agents. Compared to photocatalysis, much more semiconductors even with a lower CB level than CO2 redox potential could be function as the photocathodes. Figure 1 shows the CB, VB band edge positions versus an NHE and band gap energies for several common semiconductor photocathodes relative to CO2 reduction potentials for different products at pH = 7. The CB levels of most of the semiconductors shown in the figure are belowthesingle-electronreductionpotentialofCO toCO·−,andonlyseveralofthemareabovethe 22 thermodynamic potentials of proton-assisted multi-electron reduction in CO2.PDF Image | Advances in Clean Fuel Ethanol Production from CO2 Reduction
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