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in the present CO₂/NaOH system.

In addition, Pastrana‐Martínez et al. prepared a graphene derivative–TiO₂ composites for photocatalytic water reduction of CO₂ into renewable fuels under UV–vis light irradiation [62]. Meanwhile, the findings present that the pH of solution has significant influence toward selective ethanol formation. The prepared GO/TiO₂ composite exhibited superior photocatalytic activity for EtOH production (144.7 μmol g_cat⁻¹ h⁻¹) at pH 11.0 and for MeOH production (47.0 μmol g_cat⁻¹ h⁻¹) at pH 4.0. It is found that the yield of C2 hydrocarbons is much lower than that of C1 products due to the demand of more electrons and complicated reaction mechanism, which cannot be varied. Therefore, it is more feasible that the long‐chain hydrocarbons are formed as secondary products over photocatalysts that can realize efficient C1–C2 conversion in the future.

2.4.1.6 Other Hydrocarbons

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].

(a) Scheme of the photocatalytic CO2 reduction over CdS/(Cu–NaxH2−xTi3O7) irradiated by light. (b) Gas evolution rates of C1–C3 hydrocarbons on CdS/(Cu–NaxH2−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

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].

2.4.2 Carbon Monoxide (CO)

Apart from the hydrocarbons, other carbon‐containing fuels such as CO also are produced as main product in CO₂‐involving photocatalysis [66]. From the view of redox potential, CO formation reaction (−0.48 eV vs. NHE) is inferior to the production of CH₄ (−0.38 eV vs. NHE) and CH₃OH (−0.24 eV vs. NHE), while the formation reactions of CH₄ and CH₃OH involve six‐ and eight‐electron reaction process, in which a series of elemental reaction are unclear and slow. In contrast, carbon monoxide is simpler than CH₄, CH₃OH, and other hydrocarbons, which means fewer needs of reductive electrons. Therefore, the CO reduction reaction is thermodynamically triggered, and CO is a preferred product in CO₂ photoreduction. For instance, Miyauchi and coworkers reported CuO‐decorated Nb₃O₈ nanosheets for photocatalytic CO₂ reduced into CO, and simultaneously the reaction pathway over this system had been deeply investigated through electron spin resonance (ESR) and isotope‐labeled experiments [67]. The results indicate that amorphous copper oxide nanoclusters can work as efficient electrocatalysts grafted onto the surface of niobate nanosheets for the reduction of carbon dioxide to carbon monoxide. Furthermore, the photocatalytic activity and reaction pathway of Cu(II)‐grafted Nb₃O₈ nanosheets were investigated using ESR analysis and isotope‐labeled molecules (H₂¹⁸O and ¹³CO₂). The results of the labeling experiments demonstrated that under UV irradiation, electrons are extracted from water to produce oxygen (¹⁸O₂) and then reduce CO₂ to produce ¹³CO. ESR analysis confirmed that excited holes in the VB of Nb₃O₈ nanosheets react with water and that excited electrons in the CB of Nb₃O₈ nanosheets are injected into the Cu(II) nanoclusters through the interface and are involved in the reduction of CO₂ into CO. The Cu(II) nanocluster‐grafted Nb₃O₈ nanosheets are composed of nontoxic and abundant elements and can be facilely synthesized by a wet chemical method. The nanocluster grafting technique described here can be applied for the surface activation of various semiconductor light harvesters, such as metal oxide and/or metal chalcogenides, and is expected to aid in the development of efficient CO₂ photoreduction systems. Furthermore, Tahir et al. synthesized Ag nanoparticles/TiO₂ nanowires core–shell heterojunction for efficient photoreduction of CO₂ to CO in the presence of hydrogen [68]. Ag‐NPs coated over TiO₂ NWs exhibited strong absorption of visible light due to localized surface plasmon resonance (LSPR) excitation, trapped electrons, and hindered charge recombination rate. The synergistic effect of Ag‐NPs coated over TiO₂ NWs for CO₂ conversion was evaluated in a gas‐phase system under UV and visible‐light irradiation. The plasmonic Ag‐NPs/TiO₂ NWs demonstrated excellent photoactivity in the reduction of CO₂ into CO, CH₄, and CH₃OH under visible‐light irradiation. The results show that 3 wt% Ag‐NPs‐loaded TiO₂ NWs was found to be the most active, giving the highest CO evolution of 983 mmol g⁻¹ h⁻¹ at selectivity 98%. This amount of CO produced was 23 times more than the TiO₂ NWs and 109 times larger than the yield of CO produced over the pure TiO₂. More importantly, the quantum yield was substantially enhanced for CO evolution. The LSPR excitation and synergic effect of Ag‐NPs that can effectively accelerate the charge separation were proposed to be responsible for the enhancement of photocatalytic activity. The photostability of Ag‐NPs/TiO₂ NWs evidenced in cyclic runs for selective CO production under visible light, yet photoactivity declined over the

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