(Invited) Photocatalytic CO2 Reduction Using Water As an Electron Donor
Authors: Kudo Akihiko 
Published: December 2024

Photocatalytic CO2 reduction is a promising CCU reaction to solve resources, energy, and environment issues, and achievement of carbon neutrality. In the photocatalytic reaction, water should be an electron donor and a hydrogen source for the CO2 reduction [1]. This reaction is regarded as artificial photosynthesis because light energy is converted to storable chemical energy. In the present paper, various metal oxide and sulfide photocatalysts and semiconductor photoelectrodes for CO2 reduction using water as an electron donor are introduced and discussed.
Photocatalysts were prepared by a solid-state reaction, a polymerizable complex method, or a liquid-solid-state reaction. Cocatalysts were loaded on photocatalysts by a liquid-phase-reduction or an impregnation method. Photocatalytic CO2 reduction was carried out using a gas-flow system. CO2 gas (1 atm) was bubbled into an aqueous solution. Evolved gas products were quantified by using online gas chromatographs.
NaTaO3:Ba of a single particulate photocatalyst was active for CO2 reduction to form CO using water as an electron donor when Ag cocatalyst was loaded on the surface [2]. The Ag/NaTaO3:Ba photocatalyst gave ca. 90% of the selectivity for CO formation by the CO2 reduction accompanied with water oxidation to form O2 in an aqueous medium. As the next step, it is challenging to demonstrate CO2 reduction to form CH4 of an eight-electron reduction product using water as an electron donor. A single particulate Rh-Ru/NaTaO3:Sr photocatalyst continuously produced CH4, H2, and O2 under UV irradiation [1,3]. The selectivity for CH4 formation based on the number of reacted electrons was about 10%. An e–/h+ ratio estimated from obtained products was 1.1 and TON based on the CH4formation to Rh and Ru cocatalysts was 2.0. These results have proven that CH4 was obtained by photocatalytic CO2reduction using water as an electron donor over the Rh-Ru/NaTaO3:Sr. C2H6 and C3H8 successfully evolved accompanied with O2 evolution by further development of dual cocatalysts. Notably, about 59 % of a selectivity (electronic efficiency) was obtained for formation of hydrocarbons.
We also demonstrated photocatalytic CO2 reduction using water as an electron donor under visible light irradiation by a Z-scheme photocatalyst and a photoelectrochemical cell using bare (CuGa)0.5ZnS2 powder prepared by a flux method as a CO2-reducing photocatalyst [4]. The Z-scheme system employing the bare (CuGa)0.5ZnS2 photocatalyst and RGO-(CoOx/BiVO4) as an O2-evolving photocatalyst produced CO of a CO2 reduction product accompanied with H2 and O2in a simple suspension system without any additives under visible light irradiation and 1 atm of CO2. When a basic reagent (i.e. NaHCO3, NaOH etc.) was added into the reactant solution (H2O + CO2), the CO formation rate and the CO selectivity increased. The selectivity for the CO formation of the Z-schematic CO2 reduction reached 10 – 20% in the presence of the basic reagent even in an aqueous solution and without loading any cocatalysts on the (CuGa)0.5ZnS2metal sulfide photocatalyst. It is notable that CO was obtained accompanied with reasonable O2 evolution indicating that water was an electron donor for the CO2 reduction. The present Z-scheme system also showed activity for solar CO2 reduction using water as an electron donor. The bare (CuGa)0.5ZnS2 powder loaded on an FTO glass was also used as a photocathode for CO2 reduction under visible light irradiation. CO and H2 were obtained on the photocathode with 20% and 80% of Faradaic efficiencies at 0.1 V vs. RHE, respectively.
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