Читать книгу Solar-to-Chemical Conversion - Группа авторов - Страница 22

From the reaction procedure of CO₂ photoreduction, the reductive electrons mainly react with the adsorbed carbon species into C1 products because these reactions can be more easily driven thermodynamically. On the other hand, more valuable hydrocarbons including ethane and ethanol are more desirable such that it is inevitable to proceed multistep reactions involving more complicated reaction mechanism. It is well known that a multistep reaction means decaying of reaction efficiency and increasing of by‐products. Therefore, selectivity is a key parameter for the production of C2 or higher hydrocarbons.

In general, there are two possible ways of CO₂ reduction to C₂H₆, which is from hydrogenation of ethylene intermediate or dimerization of two CH₃ adsorbates. Recently, Choi and his group introduced a thin Nafion layer on Pd‐deposited TiO₂ nanoparticles that markedly enhances the photosynthetic conversion of CO₂ to hydrocarbons (mainly CH₄ and C₂H₆) in an aqueous suspension without any sacrificial electron donor under UV and solar irradiation conditions [57]. In this system, the Nafion layer functioned as a conductive and fixed layer to facilitate diffusion of protons onto the CO₂ reduction sites and immobilize intermediates of CO₂ reduction, which could help the serial electron transfers from the intermediate to the final product. The hybridized catalyst displays the CH₄ and C₂H₆ production of 123 and 12 μmol g_cat⁻¹, which apparently inhibited H₂ generation. Besides, Kulandaivalu et al. constructed a CQDs/Cu₂O nanocomposite photocatalyst with good optical, physical, and chemical properties that was able to selectively reduce CO₂ to C₂H₆ under visible‐light irradiation [58]. The improved photoactivity is the accumulated C₂H₆ of 101.83 μmol g_cat⁻¹ for six hours under continuous visible‐light illumination, while the pure Cu₂O photocatalyst produces C₂H₆ of 66.73 μmol g_cat⁻¹ for the same reaction time. The findings present that no methane or ethylene is detected over bare Cu₂O and CQDs/Cu₂O hybrids, indicating that the dimerization of CH₃ adsorbates is favorable in this system. The proposed step number of C₂H₆ production is eight, where both the photoinduced electrons and holes take part in the CO₂ photoreduction and 3.5 molecules of O₂ for each C₂H₆ molecular is released from water splitting process but no hydrogen is detected.

Apart from ethane, ethanol is also reported in the photoreduction of CO₂ in the past [59]. Recently, Liu et al. reported visible‐light‐responsive photocatalyst BiVO₄ for selective formation of ethanol under the condition of high‐intensity visible‐light irradiation [60]. It is known that the proton in water cannot capture the photogenerated electrons on BiVO₄ to produce H₂. In contrast, once CO₂ molecule is adsorbed and changed to CO₃²⁻, the photogenerated electrons can react with CO₃²⁻ likely and give rise to methanol after protonation. Furthermore, under intense irradiation, a large number of C1 intermediate species are anchored on the surface of BiVO₄, which will facilitate dimerization of C1 intermediate species to form ethanol. Based on this strategy, Peng and coworkers took advantage of graphitic carbon nitride with high CB position for achieving selectively photocatalytic reduction of CO₂ under visible‐light irradiation [61]. It was found that the starting materials of graphitic C₃N₄ have significant influence on the activity and selectivity of CO₂ reduction. Once graphitic carbon nitride was fabricated by using urea (denoted as u‐g‐C₃N₄) that possessed large surface area with mesoporous nanostructure, the reduced products of CO₂ were a mixture of CH₃OH and C₂H₅OH in the presence of NaOH (1.0 M), where the yields were 6.28 and 4.51 μmol g_cat⁻¹ h⁻¹, respectively. Compared with u‐g‐C₃N₄, the sample prepared by using melamine (denoted as m‐g‐C₃N₄) mainly catalyzed CO₂ to C₂H₅OH under the same conditions, where the yield of C₂H₅OH was 3.64 μmol g_cat⁻¹ h⁻¹. Meanwhile, high concentration O₂ and trace H₂ were detected in two samples, inferring that photoexcited holes at VB of C₃N₄ can be efficiently formed from water splitting and the corresponding electrons took part in the conversion of CO₂ with H⁺ into alcohols. Moreover, by comparing u‐g‐C₃N₄ with m‐g‐C₃N₄, it shows that larger surface area, smaller crystal size, and lower crystallinity may be the main reasons for the apparent difference in selective formation of CH₃OH and C₂H₅OH in the present system. Owing to the nonporous structure of m‐g‐C₃N₄, the fast exchange of the formed ·OCH₃ or CH₃OH may be suppressed, thereby in turn benefiting the dimerization to produce C₂H₅OH 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.