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As a high value‐added fuel, CH₄, a natural gas, is widely used in residential living and industrial activities. Reduction of CO₂ to methane named by the methanation process is of substantial importance in industrial field as the following equation: CO₂ + 2H₂ → CH₄ + 2H₂O. In general, this reaction is proceeded at high temperatures and pressures in the presence of metal catalysts, for example, Ru, Mo, and Ni. However, inspired by natural photosynthesis, chemists expect to produce CH₄ fuel under mild and environment‐friendly conditions. In 1986, Willner and coworker reported an exciting result, in which gaseous CH₄ was successfully synthesized with a 0.0025% of quantum yield by using a Ru(bpz)₃²⁺ as sensitizer and Ru metal colloid as catalyst under visible‐light irradiation [25]. In Figure 2.8, the reaction mechanism displayed that Ru*(bpz)₃²⁺ produced from the light‐excited Ru(bpz)₃²⁺ can be reduced by triethanolamine (TEOA) as electron donor into Ru(bpz)₃⁺ that can pair with Ru(bpz)₃²⁺ with a redox potential of −0.86 V vs. Saturated Calomel Electrode (SCE). On the other hand, it is demonstrated that the redox potential of CH₄/CO₂ is equal to −0.24 V vs. NHE, which is less negative than that of Ru(bpz)₃⁺/Ru(bpz)₃²⁺. As a result, CO₂ molecular can be thermodynamically reduced into CH₄ molecular by Ru(bpz)₃⁺ over Re metal catalyst in acidic media according to the Eq. (2.7).

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.

Figure 2.9 (a) Calculated band structures and total density of states of BMO‐OVs. Absorptive formats of CO₂ on BMO‐OVs (b) and BMO (c). CO₂ reduction pathways on BMO‐OVs (d) and BMO (e). (f) The diagram of band positions. (g) CH₄ production and the TCEN value for CO₂ photoreduction over the BMO‐OVs and BMO during four hours visible‐light irradiation. (h) Typical time course of CO/CH₄ generated over BMO‐OVs.

Source: Yang et al. [30].

Moreover, new layered photocatalytic systems have been built to provide more active sites; therefore, high reaction rate can be achieved. For example, Wang and coworkers reported in situ prepared ultrathin SiC/reduced graphene oxide (RGO) nanosheets heterojunction for high yield and selectivity of CH₄ under visible‐light irradiation [31]. The result demonstrated that ultrathin SiC nanosheets is beneficial for suppressing charge recombination and more energetic electrons can transfer to RGO for promoting CO₂ activation. However it is found that the amount of RGO added needed to be optimized due to the competitive reaction of CO/CO₂ (−0.52 V vs. NHE) with CH₄/CO₂ (−0.24 V vs. NHE). Excessive RGO in SiC/RGO will dilute the energetic electron density on RGO, which is beneficial for proceeding of two‐electron CO/CO₂ reaction and suppresses the eight‐electron process of CH₄/CO₂ despite the redox potential of CH₄/CO₂ being lower than that of CO/CO₂. Under the optimal conditions, the yield and selectivity of CH₄ reached to 6.72 μmol h⁻¹ g⁻¹ and ∼75%, respectively.