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Figure 2.8 Schematic cycle for the photosensitized reduction of CO₂ to CH₄.

      Source: Maidan and Willner [25].

However, the chemical yield of CH₄ from this expensive organic/inorganic photocatalytic system is low and unsustainable. In 1995, Kamber and coworkers took advantage of popular photocatalyst TiO₂ to reduce gaseous CO₂ into CH₄ under ultraviolet (UV) illumination at moderate temperature while the chemical yield and selectivity is not satisfying due to the nonspecific reaction over semiconductor photocatalysts [26]. The chemical yield of CO and H₂ is 27 and 14 times higher than that of CH₄, which indicated CH₄ seemed the by‐products and the formation reactions of CO and H₂ are the dominant reduction reaction.

Considering the commercial implication in future, the selectivity to CH₄ is a key challenge in transforming CO₂ into CH₄. Recently, Tasbihi et al. reported Pt/TiO₂/mesoporous SiO₂ photocatalyst for significant enhancement of selectivity to CH₄ [27]. The main motivation of constructing this system is to elucidate that the introduced platinum (Pt) not only increases the activity of TiO₂ catalysts for CO₂ reduction but also modifies the selectivity toward highly reduced products. It is reported that compared with other noble cocatalysts, such as palladium, gold, silver, and rhodium, platinum has shown exceptional selectivity to methane in CO₂ photoreduction, and, therefore, the Pt nanoparticles are to be expected to improve the selectivity of catalytic processes [28]. To further reduce the cost of photocatalysts for CH₄ production, transition metal elements are investigated to substitute noble metal as well as maintain high selectivity. For example, Zhao and coworkers prepared a surface La‐modified TiO₂ photocatalyst through a facile sol–gel route [29]. It was found that the La species was deposited on the surface of TiO₂ in the form of La₂O₃. More importantly, small part of La atoms replaced surface Ti atom forming a Ti—O—La bond, resulting in the generation of oxygen vacancies (OVs) and Ti³⁺, which synergistically contributed to the enhanced CO₂ adsorption, water vapor activation, and charge separation. Finally, the selectivity to CH₄ in La‐modified TiO₂ reached to ∼80% compared with ∼20% selectivity of TiO₂. Meanwhile, Ye and coworkers created a rich of oxygen vacancies on the (001) facet of Bi₂MoO₆ (BMO), which significantly improved the special CO₂ adsorption in a bidentate carbonate mode, thereby facilitating hydrogenation of intermediate *CO to form methane in a high selectivity of ∼88% [30], as present in Figure 2.9. Meanwhile, a broader density functional theory (DFT) study was performed to further explore the possible CO₂ conversion pathways and intermediate species initiated by these two different CO₂ adsorption configurations in Figure 2.9d,e. For the defect‐free BMO, only CO was produced in a rate of 0.12 μmol g⁻¹ h⁻¹ with total consumed electron number (TCEN) of 0.24 μmol g⁻¹ h⁻¹ in Figure 2.9g, while for BMO‐OVs, the introduced OVs served as the natural active sites, leading to remarkable production of CH₄ with rate of 2.01 μmol g⁻¹ h⁻¹ as the main product, CO of 0.27 μmol g⁻¹ h⁻¹, and TCEN of 16.62 μmol g⁻¹ h⁻¹, respectively.