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

Abstract

Conspectus 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–x Zn2x S2, CuGa1–x In x S2, 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 reducti...

Excited Electron‐Rich Bi(3–x)+ Sites: A Quantum Well‐Like Structure for Highly Promoted Selective Photocatalytic CO2 Reduction Performance

“…The increasing use of fossil fuels generates a large amount of CO 2 that can affect the global climate and solar‐driven CO 2 conversion is a promising strategy to reduce the carbon footprint. [ 1,2 ] However, the efficiency of CO 2 reduction photocatalysts is unsatisfactory on account of inefficient separation of photogenerated carriers as well as the poor capture/activation ability of the CO 2 . [ 3 ] In addition, the current exploration of photocatalytic mechanism is mainly in the research of ground‐state semiconductor materials.…”

Photocatalytic CO2 Conversion Using Metal-Containing Coordination Polymers and Networks: Recent Developments in Material Design and Mechanistic Details

“…The combination of metal and polymer moieties merges the advantage of both materials: Metal centers build up the functional core as redox-active species, whereas polymeric materials are able to optimize the catalyst performance while acting as stabilizing structures due to their feature of being tunable. Especially for carbon dioxide conversion, metal-containing polymers can support photocatalytic [ 11 , 12 , 13 , 14 , 15 ], electrocatalytic [ 16 , 17 , 18 , 19 , 20 ], photo-electrocatalytic [ 13 , 15 , 21 , 22 , 23 ] or thermochemical [ 24 ] reduction reactions.…”

Photoexcited charge manipulation in conjugated polymers bearing a Ru(ii) complex catalyst for visible-light CO2 reduction

“…[9][10][11] In particular, the development of photocatalyst materials, which facilitate CO 2 reduction without high temperature and/or pressure, has been regarded as a promising way. 7,8 For the past four decades, molecular-based photocatalysts including metal complexes 12,13 and semiconductor-based photocatalysts including metal oxides [14][15][16] and mixed-anion materials 16,17 have been extensively studied. High selectivity for CO 2 reduction, based on well-dened and tunable active sites, is one of the advantages of molecular photocatalysts, as has been demonstrated so far.…”

“…8,15 The band-gap excitation of semiconductors generates multiple electrons and holes in the conduction and valence bands. Therefore, simultaneous multi-electron reduction and oxidation reactions, e.g., overall water splitting [19][20][21][22] and CO 2 reduction using water as the electron source, 16 have been accomplished using a number of semiconductor photocatalysts, while such reactions have hardly been reported using a molecular photocatalyst system. 23 In terms of CO 2 reduction, however, semiconductor photocatalysts frequently suffer from the low selectivity of CO 2 reduction by competing with efficient proton reduction.…”

“…23 In terms of CO 2 reduction, however, semiconductor photocatalysts frequently suffer from the low selectivity of CO 2 reduction by competing with efficient proton reduction. 16 Recently, molecular-semiconductor hybrid photocatalysts have been developed to maximize their advantages, although it still remains a challenge for efficient photocatalytic CO 2 reduction. [24][25][26] Recently, organic conjugated polymers have emerged as new candidates for photocatalyst materials.…”