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As described in Eq. (2.3), HCHO is one of products that can be theoretically and experimentally formed, involving four‐electron reactions, while more elementary reactions happen during the final formation of HCHO [42]. Meanwhile, HCHO is also detected as an intermediate for fulfilling next reactions and producing CO and other advanced hydrocarbons. For example, Ojha et al. found that HCHO and COO– functioned as important intermediates to form CO over Cu‐modified S‐doped g‐C₃N₄ thin sheets through in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) [43]. Meanwhile, it is found that the existing forms of Cu in S‐doped g‐C₃N₄ system include Cu⁰, Cu⁺, and Cu²⁺ three states, which have obvious effects in the adsorbed species and reaction pathways. Owing to the higher absorbance value on the Cu⁰‐decorated sheets, the bicarbonate species as an intermediate tended to promote CH₄ production. Furthermore, Song and coworkers explore the selectivity and photocatalytic 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.