CO₂ Reduction Using Water as an Electron Donor over Heterogeneous Photocatalysts Aiming at Artificial Photosynthesis

Photocatalytic and photoelectrochemical CO₂ reduction of artificial photosynthesis is a promising chemical process to solve resource, energy, and environmental problems. An advantage of artificial photosynthesis is that solar energy is converted to chemical products using abundant water as electron and proton sources. It can be operated under ambient temperature and pressure. Especially, photocatalytic CO₂ reduction employing a powdered material would be a low-cost and scalable system for practical use because of simplicity of the total system and simple mass-production of a photocatalyst material.In this Account, single particulate photocatalysts, Z-scheme photocatalysts, and photoelectrodes are introduced for artificial photosynthetic CO₂ reduction. It is indispensable to use water as an electron donor (i.e., reasonable O₂ evolution) but not to use a sacrificial reagent of a strong electron donor, for achievement of the artificial photosynthetic CO₂ reduction accompanied by ΔG > 0. Confirmations of O₂ evolution, a ratio of reacted e⁻ to h⁺ estimated from obtained products, a turnover number, and a carbon source of a CO₂ reduction product are discussed as the key points for evaluation of photocatalytic and photoelectrochemical CO₂ reduction.Various metal oxide photocatalysts with wide band gaps have been developed for water splitting under UV light irradiation. However, these bare metal oxide photocatalysts without a cocatalyst do not show high photocatalytic CO₂ reduction activity in an aqueous solution. The issue comes from lack of a reaction site for CO₂ reduction and competitive reaction between water and CO₂ reduction. This raises a key issue to find a cocatalyst and optimize reaction conditions defining this research field. Loading a Ag cocatalyst as a CO₂ reduction site and NaHCO₃ addition for a smooth supply of hydrated CO₂ molecules as reactant are beneficial for efficient photocatalytic CO₂ reduction. Ag/BaLa₄Ti₄O₁₅ and Ag/NaTaO₃:Ba reduce CO₂ to CO as a main reduction reaction using water as an electron donor even in just water and an aqueous NaHCO₃ solution. A Rh-Ru cocatalyst on NaTaO₃:Sr gives CH₄ with 10% selectivity (Faradaic efficiency) based on the number of reacted electrons in the photocatalytic CO₂ reduction accompanied by O₂ evolution by water oxidation.Visible-light-responsive photocatalyst systems are indispensable for efficient sunlight utilization. Z-scheme systems using CuGaS₂, (CuGa)1-xZn2xS₂, CuGa1-xInxS₂, and SrTiO₃:Rh as CO₂-reducing photocatalyst, BiVO₄ as O₂-evolving photocatalyst, and reduced graphene oxide (RGO) and Co-complex as electron mediator or without an electron mediator are active for CO₂ reduction using water as an electron donor under visible light irradiation. These metal sulfide photocatalysts have the potential to take part in Z-scheme systems for artificial photosynthetic CO₂ reduction, even though their ability to extract electrons from water is insufficient.A photoelectrochemical system using a photocathode is also attractive for CO₂ reduction under visible light irradiation. For example, p-type CuGaS₂, (CuGa)1-xZn2xS₂, Cu1-xAgxGaS₂, and SrTiO₃:Rh function as photocathodes for CO₂ reduction under visible light irradiation. Moreover, introducing a conducting polymer as a hole transporter and surface modification with Ag and ZnS improve photoelectrochemical performance.