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

Abstract

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-xZn_2xS₂, CuGa_1-xIn_xS₂, 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-xZn_2xS₂, Cu_1-xAg_xGaS₂, 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.

Figure 1

Artificial photosynthetic CO₂ reduction based on a powdered photocatalyst by (a) a single-particulate system, (b) a Z-scheme system, (c) a photoelectrochemical system using a photocathode, and (d) a photoelectrochemical system combining a photocathode and a photoanode.

Figure 2

(A) SEM images of Ag/BaLa₄Ti₄O₁₅ before and after photocatalytic CO₂ reduction, and the proposed mechanism. (B) Photocatalytic CO₂ reduction using water as an electron donor under UV light irradiation over Ag(2 wt %)/BaLa₄Ti₄O₁₅. Photocatalyst, 0.3 g; reactant solution, water (360 mL); flow gas, CO₂ (1 atm); light source, 400 W high-pressure mercury lamp; reaction cell, inner irradiation quartz cell. Reproduced with permission from ref (1). Copyright 2011 American Chemical Society.

Figure 3

(A) Photocatalytic CO₂ reduction using water as an electron donor under UV light irradiation over Ag/NaTaO₃:Ba. Reactant solution, NaHCO_3(aq) (360 mL); flow gas, CO₂ (1 atm); light source, 400 W high-pressure mercury lamp; reaction cell, an inner irradiation quartz cell. (B) Proposed mechanism of photocatalytic CO₂ reduction in the presence of NaHCO₃. Reproduced with permission from ref (2). Copyright 2017 Wiley.

Figure 4

(A) Various types of Z-scheme photocatalysts for CO₂ reduction using water as an electron donor. (B) Z-scheme CO₂ reduction under visible light irradiation using CuGaS₂ or (CuGa)_0.5ZnS₂ prepared by a SSR or a flux method combined with RGO–(CoO_x/BiVO₄). Reproduced with permission from ref (4). Copyright 2022 American Chemical Society. (C) Z-scheme CO₂ reduction under visible light irradiation using [Ru(dpbpy)]/(CuGa)_0.3Zn_1.4S₂, BiVO₄, and [Co(tpy)₂]^3+/2+. Reproduced with permission from ref (43). Copyright 2018 The Royal Society of Chemistry. Photocatalyst, 0.1–0.4 g; reactant solution, NaHCO_3(aq) (120–150 mL); flow gas, CO₂ (1 atm); light source, 300 W Xe lamp (λ > 420 nm); irradiation area, 33 cm².

Figure 5

(A) Two-electrode and three-electrode systems for photoelectrochemical CO₂ reduction. (B) Photoelectrochemical CO₂ reduction under visible light irradiation over a (CuGa)_0.5ZnS₂ powder-based photocathode. Electrolyte, 0.1 mol L^–1 KHCO_3(aq); flow gas, CO₂ (1 atm); light source, 300 W Xe lamp (λ > 420 nm); applied bias, 0.1 V vs RHE (−0.5 V vs Ag/AgCl (pH 6.9)). Reproduced with permission from ref (4). Copyright 2022 American Chemical Society.