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Apart from these hydrocarbons produced in CO₂ photoreduction with high frequency, some complicated hydrocarbons with more than two carbon numbers have been reported recently. Fusco et al. reported firstly the formation of C2 hydrocarbons, acetic acid, through CO₂ photoreduction over TiO₂@PEI‐grafted‐MWCNTs hybrids under UV–Vis light irradiation [63]. Besides, Park et al. decorated Cu nanoparticles and CdS quantum dots on the surface of TiO₂ nanotubes, forming a ternary nanostructured photocatalyst that is capable of converting CO₂ and H₂O into C1–C3 hydrocarbons, including CH₄, C₂H₆, C₃H₆, and C₃H₈ (Figure 2.12b–e) [64]. In this system, CdS quantum dots are responsible of harvesting solar light and forming hot electrons that will rapidly move to Cu nanoparticles through TiO₂ nanotubes. At Cu nanoparticles, the concentrated electrons can react with CO₂ molecular and produce C_xH_y under visible‐light excitation (above 420 nm), as shown in Figure 2.12a. In contrast, over five hours irradiation, free H₂ molecular that is more easily formed than CO₂ reduction products was not detected, which inferred that competitive H₂ evolution reaction has been suppressed efficiently in this CdS/(Cu–TNTs) hybrid system. The photocatalytic reduction of CO₂ over the CdS/(Cu–TNTs) hybrid is initiated likely through a one‐electron reduction to form ˙CO₂⁻, reacting in turn with a H atom (H·) to produce hydrocarbons (Figure 2.12f). CO₂ hydrogenation leading to hydrocarbon formation appears to be a less likely pathway because of the absence of H₂ detected in the headspace of the photolysis reactor during five hours of irradiation with visible light. However, the yield and efficiency are sharply decreased owing to involvement of more electrons, formation of longer C–C chains, and production of more by‐products, which severely undermine the proportion of expected products, such as C3 or C4 hydrocarbons. Therefore, to increase the formation possibility and efficiency of major hydrocarbons, electrocatalytic CO₂ reduction is paid attention through introducing more strong hot electrons to react with adsorbed CO₂ molecular over a variety of electrocatalysts, leading to longer carbon chain hydrocarbons [65].

Figure 2.12 (a) Scheme of the photocatalytic CO₂ reduction over CdS/(Cu–Na_xH_2−xTi₃O₇) irradiated by light. (b) Gas evolution rates of C1–C3 hydrocarbons on CdS/(Cu–Na_xH_2−xTi₃O₇) as well as the specific surface area, where the Na/Ti ratios were 0.093 (low), 0.143 (medium), and 0.507 (high). (c–e) Mass spectra of the formed hydrocarbons (methane, ethane, and propane) with the labeling of ¹³C. (f) Proposed elementary reaction mechanisms of photocatalytic CO₂ conversion into hydrocarbons.

Source: Park et al. [64].