Heterogeneous Photocatalyst for CO₂ Reduction

Photocatalytic CO₂ reduction is one of the representative artificial photosynthetic reactions in which light energy is converted to chemical energy as storable chemicals. CO₂ can be used as a carbon source to create a route of a carbon cycle through the photocatalytic reaction. In this reaction, water has to be used as a hydrogen source and an electron donor accompanied with formation of oxygen molecules. In Sects. 45.2 and 45.3, basic points of photocatalytic CO₂ reduction are explained. Schemes of the photocatalytic CO₂ reduction using single-particulate and two-particulate systems consisting of heterogeneous photocatalysts (semiconductor photocatalysts) are indicated. The mechanism is explained in detail using the single-particulate system. Points that should be paid attention are stoichiometry of products, a carbon source, durability of the reaction, a turnover number, photoresponse, and efficiencies of an apparent quantum yield and solar energy conversion. Those points are mentioned to evaluate correctly the photocatalytic reaction. In Sect. 45.4, single-particulate metal oxide photocatalysts that are active for CO₂ reduction using water as an electron donor are introduced. Effects of Ag cocatalyst and hydrogencarbonate on the photocatalytic activities, and the reaction mechanism are discussed using NaTaO₃:Ba with perovskite structure, BaLa₄Ti₄O₁₅ with layered perovskite structure, and KCaSrTa₅O₁₅ with tungsten bronze structure as a case study of photocatalytic CO₂ reduction. In Sect. 45.5, visible light responsive metal sulfide photocatalysts are introduced for sacrificial CO₂ reduction that is a half reaction of CO₂ reduction of an uphill reaction using water as an electron donor. Then, Z-scheme systems consisting of the metal sulfide photocatalyst, CuGaS₂, as a CO₂ reducing photocatalyst, TiO₂ and BiVO₄ as an O₂-evolving photocatalyst, and a reduced graphene oxide (RGO) as a solid-electron mediator are shown. The CuGaS₂-BiVO₄-RGO system works for photocatalytic CO₂ reduction under visible light using water as an electron donor in a simple suspension system without any additives.