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As discussed above, an ideal artificial photosynthesis system usually contains at least three different components: light harvesting, water oxidation, and CO₂ reduction [74]. To achieve CO₂ photoreduction accompanied with water oxidation, early single integrated systems are composed of semiconducting metal oxide with large band gaps absorbing UV light and metal or metal oxide cocatalysts, where considerable efforts are paid to functionalizing these materials and exploring various modifications [75]. Nevertheless, since the assembly of multifunctional units in an integrated device is extremely difficult, an alternative strategy is to divide the overall process into two half‐reactions: water oxidation and CO₂ reduction. Once each half‐reaction is well understood and optimized, the two reactions can be coupled in an integrated device. For example, it is demonstrated that combining two semiconductor materials in tandem (Z‐scheme) mode can efficiently extend the light response into the visible region [76]. Recently, Arai et al. reported a photoelectrochemical system consisting of SrTiO₃ photoanode for water oxidation and InP photocathode for CO₂ reduction that produced O₂ and formate, respectively [77]. Besides, mononuclear [Ru(bpy)₃]²⁺ is often combined with S₂O₈²⁻ sacrificial acceptor to evaluate water oxidation photocatalysis systems in aqueous photosensitization system in the past decades [78]. For example, upon combining mild oxidant [Ru(bpy)]₃³⁺ of well‐defined potential (+1.26 V) with a Co₃O₄ nanotube in neutral aqueous solution, a hole can be efficiently transferred to the catalyst [79]. Subsequently, four holes transfer from [Ru(bpy)₃]³⁺ to Co₃O₄ and react with two H₂O molecules to O₂ and 4H⁺, which indicates that Co₃O₄ is appropriate candidate for use in an artificial photosynthetic assembly.

On the water oxidation side, several groups proposed approaches to overcome challenges of efficiently combining molecular light absorbers with multi‐electron water oxidation catalyst inspired by the tyrosine mediator design of nature's photosystem II. For instance, the Mallouk group reported a series of researches on coupling of Ir oxide nanocluster (IrO_x) with [Ru(bpy)₃]²⁺ or a porphyrin visible‐light absorber through a benzimidazole–phenol redox linking with both components covalently anchored on a TiO₂ surface [80]. Based on the results of transient optical spectroscopy, when the Ru chromophore absorbs a visible photon, an photoinduced electron injects into TiO₂ at an ultrafast speed, and subsequently an electron transfers from the benzimidazole–phenol mediator to the oxidized Ru complex for reducing the oxidized chromophore on a short time scale compared to the catalytic turnover of O₂ at the IrO_x particle. As a result, the competition between undesired transfer of electrons injected into TiO₂ back to the oxidized light absorber and hole injection into the IrO_x catalyst is shifted in favor of catalysis, thus improving the quantum efficiency of water oxidation by a factor of three. The approach offers substantial room for further efficiency improvement because the relative position of the light absorber and the catalyst with the attached mediator is not yet molecularly defined in the present system.

Recently, earth‐abundant inorganic oxide catalysts have been widely used in water oxidation, accompanied with CO₂ photoreduction. In the past decades, Ir oxide nanoclusters have been viewed as efficient catalysts for water oxidation, which can be used to assemble directly various fashion systems for closing the photosynthetic cycle on the nanoscale. For example, Kim et al. prepared an all‐inorganic polynuclear unit consisting of an oxo‐bridged binuclear ZrOCo^II group with iridium oxide nanocluster assembled on SBA‐15 silica mesopore surface, which could produce CO and O₂ simultaneously [81]. Apart from Ir‐based cocatalyst, other noble metals also are applied into the CO₂ photoreduction and water oxidation by deposition on semiconducting materials. For example, Fan et al. fabricated a binary functional photocatalyst by loading Au@Pd nanoparticles onto oxygen vacancy‐rich TiO₂ through surfactant‐free deposition–reduction method [82]. The obtained V_O‐rich TiO₂ contributes more protons to water oxidation process, and meanwhile, the metallic Au@Pd sites are beneficial for CO₂ activation and proton supply. By varying the ratio of Au@Pd nanoparticles and V_O concentration, CH₄ selectivity of 96% can be obtained with minimal H₂ production. However, the Ir oxide or other noble metals are not a viable component for a scalable artificial photosystem because of the scarcity of this element. Therefore, earth‐abundant metal oxides are developed as robust alternatives. For example, Yu et al. Cl‐doped Cu₂O nanorods for photocatalytic CO₂ reduction conjugated with H₂O oxidation under visible‐light irradiation [83]. Owing to the introduction of Cl atoms, the band structure of Cu₂O is optimized, leading to a more positive VB position for water oxidation, which promotes CO₂ activation and separation and transfer efficiency of photoinduced charge carriers. As a result, in Figure 2.14, the Cl‐doped Cu₂O composite exhibits excellent photocatalytic CO₂ reduction performance accompanied by favorable water oxidation ability under visible‐light irradiation. DFT calculations demonstrate that the doping of Cl atoms facilitates to transform CO₂ into the intermediates of *COOH, *CO, and *CH₃O, which could enhance production of reductive CO and CH₄ and oxidative O₂. The CO, CH₄, and O₂ amount were improved to 1.74 μmol cm⁻², 0.39 μmol cm⁻², and 1.32 μmol cm⁻², respectively, for the Cu₂O–Cl‐4, as shown in Figure 2.14a. Furthermore, it is found that the Cl‐doped Cu₂O composite displays stronger affinity toward the *CO intermediate, which tends to be protonated and ultimately produce CH₄, leading to higher selectivity of CH₄ than that of pure Cu₂O.

Figure 2.14 (a) Gas production under six hours visible‐light irradiation of pure Cu₂O and Cl‐doped Cu₂O samples. (b) Mass spectrum from the ¹⁸O‐labeling H₂O isotope experiments of Cu₂O–Cl‐4. (c) Time‐dependent product evolution over Cu₂O–Cl‐4 under visible‐light irradiation. (d) Stability test of Cu₂O–Cl‐4. (e) Proposed reaction pathways of CO₂RR to CO and CH₄ on Cl‐doped Cu₂O.

Source: Yu et al. [83].

Based on early reports of electro‐ or light‐driven Co and Mn oxide catalysts, the findings display that these catalysts have the TOFs per metal center in the range 10⁻⁴ to 10⁻² O₂ s⁻¹ at overpotentials in the 30–400 mV range depending on the structure, pH, and temperature conditions, which are at least 2 orders of magnitude lower than TOF of IrO₂ clusters at similar overpotential [84]. In the past several years, therefore, intense efforts have been paid to enhance the TOF per projected area of the catalyst by optimizing the Co‐, Mn‐, and Ni‐based metal oxide. For example, amorphous Co species (a‐Co‐E) synthesized through thermal treatment of a Co‐ethylene diamine tetraacetic acid complex were combined with Bi₂WO₆, and the product exhibited an impressively higher photocatalytic O₂ evolution rate than that of the CoO_x‐loaded counterparts [85]. The findings suggested that the photocatalytic O₂ production rate of the Bi₂WO₆/a‐Co‐E hybrid is apparently enhanced to 352.3 μmol g⁻¹ h⁻¹ with no obvious decrease in six hours, which is 3.6 times higher than that of Bi₂WO₆/CoO_x, testifying the excellent capability of a‐Co‐E to function as the cocatalyst over CoO_x. Graphitic carbon nitride and cubic cobalt manganese spinel (c‐CoMn₂O₄) as light transducer and water oxidation cocatalyst were used to assemble effective water oxidation system [86]. Owing to acceleration of interface charge transfer rate and decreasing of the excessive energy barrier for O–O formation, the oxygen evolution rate of the g‐C₃N₄/CoMn₂O₄, 18.3 μmol h⁻¹, is four times higher than that of pristine g‐C₃N₄. To further provide more active sites for water oxidation, novel mesoporous MOFs are utilized as a host material to realize the strategy [87]. Co‐based POM possessing stable water oxidation on central Co^III sites is coupled with MIL‐100(Fe), which not only immobilizes homogeneous Co‐POM cluster but also contributes more surface areas [88]. The results reveal that a 1.72‐fold increasing of oxygen yield and TOF of 9.2 × 10⁻³ s⁻¹ are achieved for the Co‐POM/MIL‐100(Fe) hybrids.