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Formic acid, the two‐electron reduction product of CO₂, has recently attracted attention as a storage source of H₂. Formic acid itself is an important chemical. It has been employed as a preservative and an insecticide and is also a useful acid, reducing agent, and source of carbon in synthetic chemical industries. Analyzing the reduction reaction of CO₂ to HCOOH, as shown in Eqs. (2.8, 2.10), it is found that this reaction involves multiple steps and multiple electrons so that the efficiency and yield are relatively low and the by‐products undermine the selectivity of HCOOH through a series of competitive reactions. Therefore, unique photocatalysts or photoelectrocatalysts should be developed to meet the need of CO₂ conversion to HCOOH:

(2.8)

(2.9)

(2.10)

Recently, bimetallic‐graphene (Pt@Au@rGO) system has been reported by Kim and coworkers, in which the hybridized system is composed of gold nanoparticles (30 nm) as the core and ultrasmall platinum nanoparticles as the satellites wrapped by RGO [49]. The results demonstrate that the noble metal nanoparticles can efficiently improve the selectivity for HCOOH formation because noble metal can decrease the overpotential of hydrogen production from water splitting, therefore beneficial to the hydrogenation of CO₂, in particular Pt nanoparticles compared with other noble metals. On the other hand, it is reported that CB of g‐C₃N₄ has enough high position, located at −1.23 V, which is more negative than the reduction potential of CO₂/CH₃OH (−0.38 V) and CO₂/HCOOH (−0.61 V). Therefore, it is often viewed as an effective photocatalyst for CO₂ photoreduction to methanol and formic acid. For example, Adekoya et al. constructed g‐C₃N₄/(Cu/TiO₂) nanocomposites through coupling g‐C₃N₄ with Cu‐modified TiO₂ to suppress the rapid recombination of photoinduced charge carriers in pristine g‐C₃N₄ [50]. As a result, the nanocomposite photocatalyst displayed enhanced photoreduction abilities of CO₂ to CH₃OH and HCOOH under UV and visible‐light irradiation. Meanwhile, the yield of HCOOH (5069 μmol g_cat⁻¹) is much higher than that of CH₃OH (2574 μmol g_cat⁻¹), but the selectivity is not good. According to the analysis of HCOOH formation and related literatures, it seems that higher CB potential and lower overpotential hydrogen facilitate the yield of HCOOH, while it is observed that the by‐product CH₃OH is higher, which is a strong competitive reaction due to the lower redox potential compared with that of HCOOH synthetic reaction [51]. Consequently, only introducing new component for enhanced charge separation and solar harvesting is hardly to achieve high selectivity of HCOOH. Therefore, novel photocatalysts with new reaction path of HCOOH should be considered and developed.

In 1992, Matsuoka et al. reported a oligo(p‐phenylenes) organic molecular photocatalyst for CO₂ photoreduction to formic acid with small quantity of CO in a nonaqueous solvent under UV light illumination [52]. To further improve the selectivity of formic acid and efficiency, Tamaki and coworkers designed a series of Ru(II) supermolecular photocatalysts that exhibit high electivity and turnover frequency (TOF) under visible‐light irradiation [53]. By means of combining photosensitizer units and catalyst units, a trinuclear complex displayed 0.061 of formic acid yield, 671 of TON, and 11.6 min⁻¹ of TOF. Recently, Tamaki et al. prepared a novel sacrificial electron donor, 1,3‐dimethyl‐2‐(o‐hydroxyphenyl)‐2,3‐dihydro‐1H‐benzoimidazole, for efficient supermolecular photocatalyst, further increasing reduction efficiency of CO₂ to HCOOH, where the TON_HCOOH and TOF_HCOOH reached to 2766 and 44.9 min⁻¹, respectively, under visible‐light irradiation [54].

Owing to the high cost and unrecyclable properties of homogeneous photocatalysts, chemical scientists pay more attention to combine solid semiconductor photocatalysts with liquid organic supermolecular photocatalysts together to overcome these obstacles as well as maintain high efficiency and selectivity. For instance, Suzuki et al. reported that a hybrid photocatalyst that consisted of Ru(II) complex and N‐doped Ta₂O₅ anchored with an organic group was fabricated through a direct assembly route [55]. In Figure 2.11a, when the N‐Ta₂O₅ was excited by suitable light source, the photogenerated electrons at the CB of N‐Ta₂O₅ transferred to the Ru complex, in which the two‐electron reduction of CO₂ to HCOOH can be easily triggered. Nevertheless, the solid semiconductor and organics should be assembled by a certain organic ligand to realize electron transfer process. To reach the goal, phosphonate and carboxylic groups were introduced to link the inorganic and organic components. The inorganic/organic system modified by phosphonate or carboxylic groups exhibited excellent photoconversion activity of CO₂ to formic acid under visible‐light irradiation with respect to the reaction rate and stability, as shown in Figure 2.11b. On the other hand, the rapid and efficient electron transfer can be achieved by adjusting the position of CB of semiconductor. For example, recently, Maeda and coworkers developed a Ru(II) complex/C₃N₄ hybrid photocatalyst for improving the photocatalytic reduction efficiency of CO₂ to HCOOH (selectivity >98%) under longer wavelength visible (λ > 500 nm) through a copolymerization preparation method, as shown in Figure 2.11c–d [56]. For the copolymerized C₃N₄ nanosheets fabricated by combing urea with phenylurea in high temperature, the absorption edge can be extended to 650 nm, which can significantly enhance photocatalytic activity and selectivity of modified Ru(II) complex/C₃N₄ system to HCOOH under longer‐wavelength light source irradiation.

Figure 2.11 (a) Mechanism for the reduction of CO₂ by photocatalysis under visible light with a Ru complex and an N‐Ta₂O₅ hybrid catalyst. (b) Turnover number for HCOOH formation from CO₂ over various molecular systems as a function of irradiation time. (c) Schematic mechanism of visible‐light‐driven photocatalytic CO₂ reduction over RuP/NS‐C₃N₄ hybrids. (d) Time courses of photocatalytic CO₂ reduction using RuP/NS‐C₃N₄‐PU0 and RuP/NS‐C₃N₄‐PU90 under visible light (λ > 400 nm).

Source: (a) Suzuki et al. [55]; (b) Sato et al. [55]; (c) Tsounis et al. [56]; (d) Tsounis et al. [56].