A Molecular Z‐Scheme Artificial Photosynthetic System Under the Bias‐Free Condition for CO₂ Reduction Coupled with Two‐electron Water Oxidation: Photocatalytic Production of CO/HCOOH and H₂O₂

Abstract

Bio‐inspired molecular‐engineered systems have been extensively investigated for the half‐reactions of H2O oxidation or CO2 reduction with sacrificial electron donors/acceptors. However, there has yet to be reported a device for dye‐sensitized molecular photoanodes coupled with molecular photocathodes in an aqueous solution without the use of sacrificial reagents. Herein, we will report the integration of Sn(IV)‐ or Al(III)‐tetrapyridylporphyrin (SnTPyP or AlTPyP) decorated tin oxide particles (SnTPyP/SnO2 or AlTPyP/SnO2) photoanode with the dye‐sensitized molecular photocathode on nickel oxide particles containing [Ru(diimine)3]2+ as the light‐harvesting unit and [Ru(diimine)(CO)2Cl2] as the catalyst unit covalently connected and fixed within poly‐pyrrole layer (RuCAT‐RuC2‐PolyPyr‐PRu/NiO). The simultaneous irradiation of the two photoelectrodes with visible light resulted in H2O2 on the anode and mainly HCOOH with CO and H2 as minor products on the cathode with high Faradaic efficiencies in purely aqueous conditions without any applied bias is the first example of artificial photosynthesis with only two‐electron redox reactions.

Photocatalytic Hydrogen Evolution from a Transparent Aqueous Dispersion of Quantum-Sized SnO2 Nanoparticles: Effect of Electron Trap Density within One Particle

“…Artificial photosynthesis as chemical conversion of solar energy should be one of the most promising approaches for a sustainable renewable energy system. − Based on the three milestones of modern artificial photosynthesis such as (1) the Honda–Fujishima effect of TiO 2 on photochemical water splitting into H 2 /O 2 , (2) oxygen evolution through chemical water oxidation catalyzed by a Ru-dimer complex reported by Meyer’s group, and (3) UV light-induced reduction of CO 2 by a Re complex reported by Lehn’s group, , extensive efforts have been made in chemical conversion coupled with a solar cell, direct water splitting by a semiconductor, − and a molecular catalyst-sensitized system (MCSS). − Efficient hydrogen evolution on the semiconductor reported by Domen’s group has promised its further development in visible light utilization. − For a practical chemical conversion of solar energy in the present stage, how visible light could be utilized should be one of the central subjects to be developed. Because of facile tuning with the required visible light absorption, molecular catalysts (MCs) stand on the most privileged position for the purpose.…”

“…Before exhibiting catalytic water oxidation, MCs in the higher oxidation states (+1, +2, +3, and +4) would decompose/transform to lose the activity. To get through/avoid this “photon-flux density problem of sunlight”, we have been challenging to develop an alternative approach of one-photon-induced two-electron water activation and have found visible light-induced water activation by metalloporphyrins such as the epoxidation of alkene with water, − water splitting into H 2 O 2 /H 2 in metalloporphyrin/semiconductor complexes with TiO 2 as an electron-conductive wire, and simultaneous photoelectrochemical CO 2 reduction on NiO coupled with H 2 O 2 formation on SnO 2 . Since metalloporphyrins serve as both molecular catalysts and photosensitizers in the systems, we termed them as “molecular catalyst sensitized systems (MCSS)”.…”

“…To get through/ avoid this "photon-flux density problem of sunlight", we have been challenging to develop an alternative approach of onephoton-induced two-electron water activation and have found visible light-induced water activation by metalloporphyrins such as the epoxidation of alkene with water, 22−35 water splitting into H 2 O 2 /H 2 in metalloporphyrin/semiconductor complexes with TiO 2 as an electron-conductive wire, 17 and simultaneous photoelectrochemical CO 2 reduction on NiO coupled with H 2 O 2 formation on SnO 2 . 18 Since metalloporphyrins serve as both molecular catalysts and photosensitizers in the systems, we termed them as "molecular catalyst sensitized systems (MCSS)". We believe that onephoton-induced two-electron water oxidation coupled with H 2 evolution/CO 2 reduction would be one of the most promising approaches to practical artificial photosynthesis.…”