Solar-to-Chemical Conversion. Группа авторов

activity of Cu‐ or Ni‐doped TiO₂ catalysts by means of a hydrolysis method for the reduction of CO₂ under irradiation from Hg lamps in a CO₂/NaOH aqueous solution [44]. The results display that CO, HCHO, CH₃OH, and CH₄ can be yielded simultaneously in Cu/TiO₂ and Ni/TiO₂ system, while methane is identified as the major product. At the optimum value of 1%, the CO₂ conversion ability and light utilization ability reach to 29.43 μmol g⁻¹ and 244.72 μmol g_cat⁻¹, respectively, which is almost seven times that of pure TiO₂ after 24 hours UV illumination. In Ni‐doped TiO₂, the maximum value of the CO₂ conversion ability and light utilization ability, amounting to 43.97 and 460.30 μmol g_cat⁻¹, respectively, was observed after 24 hours UV irradiation. The CO₂ conversion ability can be indirectly defined as the sum of the mole number of all independent carbon products, including CH₃OH, CO, HCHO, and CH₄. And meanwhile, the transformation of the photon is used to evaluate the light utilization ability, which is determined not only by the total quantity of carbon products produced but also by the H₂ yield. Therefore, the light utilization ability is the sum of the CO₂ conversion ability and the H₂ yield, which indicates that H₂ is the main product instead of reduced hydrocarbons. Recently, Garay‐Rodríguez et al. reported a Ba₃Li₂Ti₈O₂₀/CuO composite for selective reduction of CO₂ to formaldehyde under visible‐light irradiation [45]. The highest formaldehyde production is 11.6 μmol g_cat⁻¹ h⁻¹, which is mainly attributed to lower particle size distribution and higher surface area compared with others, allowing an increase in the CO₂ adsorption.

On the other hand, owing to the ability to uptake and release electron, polyoxometalate (POM) can prolong the lifetime of the intermediate by directing the flow of the charge carriers, which enhances the efficiency of photocatalytic activity in CO₂ reduction [46]. For example, Barman et al. prepared a Mo‐based polyoxometalate for selective formation of formaldehyde in photochemical carbon dioxide reduction [47]. In this system, a maximum of 0.858 mmol of formaldehyde and 0.63 mmol of formic acid were obtained in the reactions with 0.01 μmol of catalyst loading. The overall maximum turnover number (TON) for photochemical carbon dioxide reduction is 546. Meanwhile, the results present that water as an electron donor releases O₂, electrons, and protons simultaneously through water oxidation process that is a pH‐dependent reaction. A plausible mechanism of HCOOH formation is proposed, where CO₂ attached on Mo‐based polyoxometalate is converted to CO₂ anion radical under the attack of the electron and subsequently is reduced to HCOO– and formic acid in the presence of large protons. Finally, formic acid is detached to vacate the active site to further produce formaldehyde.

      To further improve the selectivity of HCHO in CO₂ photoreduction, the reaction parameters and reactors have been studied in the past, as similar as other chemical reactions. Recently, Brunetti et al. reported a continuous operating mode using C₃N₄–TiO₂ photocatalyst embedded in a dense Nafion matrix, where reaction pressure as a driving force was specifically and systematically investigated to determine reactor performance and product removal in photocatalytic CO₂ reduction [48]. The results showed that under the high feed pressure (5 bar), HCHO is the predominant production and MeOH is the secondary, where the carbon conversion efficiency achieved is 61 μmol g_cat⁻¹ h⁻¹, better performing than other photocatalytic membrane reactors. Meanwhile, the effects of H₂O/CO₂ feed molar ratio and contact time on the membrane reactor performance were explored. Total converted carbon instead did not vary significantly with reaction pressure.

      2.4.1.4 Formic Acid (HCOOH)

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