ACS: Visible light photocatalytic REDUCTION of CO2 with water as electron donor
First author: Shunya Yoshino Corresponding author: Akihiko Kudo Corresponding Author: Tokyo University of Science DOI:https://doi.org/10.1021/jacs.1c12636 full-text quick reference by using Z – scheme electrochemical cell, photocatalyst and light with water as electron donor has realized the photocatalytic reduction of CO2 under visible light irradiation,Bare (CuGa)0.5ZnS2 prepared by flux method was used as a photocatalyst for CO2 reduction.In z-Scheme system, bare (CuGa) 0.5ZNS2 photocatalyst and RGO-(CoOx/BiVO4) were used as photocatalysts for O2 generation. In a simple suspension system without any additives, CO of CO2 reduction product was produced under the condition of visible light irradiation and 1atm CO2, accompanied by H2 and O2.When alkali salts (NaHCO3, NaOH, etc.) are added to the reactant solution (H2O + CO2), the rate of CO formation and CO selectivity increase.The same alkali-salt effect was observed in the sacrifice CO2 reduction reaction with SO32- as electron donor on bare (CuGa)0.5ZnS2 photocatalyst.On the (CuGa)0.5ZnS2 metal sulfide photocatalyst, the reduction of CO2 to CO has a selectivity of 10-20% even in aqueous solution in the presence of alkali and salt.It is worth noting that the formation of CO is accompanied by the precipitation of O2, indicating that water is the electron donor in the process of CO2 reduction.In addition, the z-Scheme system with water as electron donor also shows the activity of solar CO2 reduction.Bare (CuGa) 0.5ZNS2 powder loaded on FTO glass can also be used as a photocathode for CO2 reduction under visible light irradiation.At 0.1V vs RHE, 20% and 80% Faraday efficiency CO and H2 are obtained at the photocathode, respectively.In order to solve resource, energy and environmental problems, beneficial CO2 sequestration technologies are urgently needed, especially using renewable energy sources.Photocatalytic CO2 reduction with water as electron donor is a promising reaction to convert solar energy into CO, HCOOH and CH4, known as artificial photosynthesis.Photocatalytic reduction of CO2 through artificial photosynthesis has significant potential because chemical products such as CO are obtained directly from CO2 using sunlight and water, and CO2 is a chemically stable and abundant resource.The key issues of photocatalytic CO2 reduction in artificial photosynthesis are as follows: high activity and selectivity, visible light response, use of water as electron donor, persistence and simple water suspension system.In order to realize CO2 transformation in artificial photosynthesis, the Change of Gibbs free energy (δ G) of the reaction must be considered.Although the photocatalytic CO2 reduction can be effectively carried out using sacrificial electron donors (i.e. triethanolamine and sulfites), the reaction is not artificial photosynthesis due to δ G < 0, as shown in Figure 1(a).In order to achieve the artificial photosynthesis with δ G > 0, water must be used as the electron donor to reduce CO2, and O2 is produced along with the oxidation of water.As shown in Figure 1(b) and (c), Z-Scheme photocatalyst and photoelectrochemical cell are attractive systems that use water as electron donor to achieve CO2 reduction.In addition, these systems can be widely used in visible-light responsive photocatalysts that efficiently utilize sunlight.In the past 10 years since the discovery of Ag/BaLa4Ti4O15 photocatalyst, efficient photocatalytic reduction of CO2 using water as electron donor has been achieved in uv response systems using wide-band gap photocatalyst.However, visible light and sunlight cannot be used due to the wide band gap.On the other hand, in visible-light response systems, there are few suspended photocatalysts with water as electron donor, which are active for CO2 reduction.Therefore, an efficient CO2 reduction system with visible light response needs to be constructed at this stage.Ideally, the photocatalytic CO2 reduction system should consist of a powdered photocatalyst material for simple and low-cost practical applications.In this context, metal sulfide powder photocatalysts were studied as photocatalysts for CO2 reduction in Z-Scheme system and photocathodes in photoelectrochemical system.Figure 1. (a) Downslope reaction of CO2 reduction on (CuGa) 0.5ZNS2 photocatalyst with sacrificial electron donor, (b) upslope reaction of CO2 reduction on Z-Scheme photocatalytic system consisting of (CuGa) 0.5ZNS2 and RGO-(CoOx/BiVO4).(c) Photochemical system using (CuGa) 0.5ZnS2 photocathode with water as electron donor.In photochemical cells, an external bias voltage below the REDOX potential of CO2 and H2O should be applied for energy conversion.Table 1 Effects of salt addition on z-type CO2 reduction under visible light irradiation, naked (CuGa) 0.5ZNS2 and RGO-(CoOx/BiVO4) as photocatalyst A.Figure 2. Z-type CO2 reduction using (CuGa) 0.5zns2 and RGO-(CoOx/BiVO4) photocatalysts under visible light irradiation :(a) without any additives, (b) with 10 mmol l-1 NaHCO3, and (c) with 100 mmol l-1 NaHCO3.Figure 3. Z-type CO2 reduction in the presence of 10 mmol L-1 NaHCO3 in the Z-Scheme system consisting of CuGaS2 or (CuGa) 0.5ZNS2 as reduction photocatalyst and RGO-(CoOx/BiVO4) photocatalyst under visible light.CuGaS2 was prepared by solid state reaction (SSR) at 873 K for 10 h, (CuGa) 0.5ZNS2 was prepared by solid state reaction (SSR) at 1073 K for 10 h and by flux method at 723 K for 15 h, respectively.Figure 4. Z-type CO2 reduction of (CuGa) 0.5ZNS2 and RGO-(CoOx/BiVO4) photocatalysts prepared by flux method in the presence of 10 mmol L-1 NaHCO3 under visible light irradiation.Table 2 Effects of pH values on sacrificial CO2 reduction and hydrogen evolution under visible light irradiation from bare (CuGa) 0.5ZNS2 in aqueous solution containing K2SO3 as sacrificial reagent as photocatalyst A.Figure 5 sacrificial CO2 reduction from (CuGa) 0.5zns2 photocatalyst containing K2SO3 as sacrificial reagent under visible light irradiation :(a) without NaHCO3, (b) with 1.0 mol l-1 NaHCO3.Figure 6. (a) Diffuse reflectance spectra of 0.5ZnS2 before CO2 reduction (CuGa), 0.5ZnS2 after CO2 reduction (CuGa) with (b) no NaHCO3 and (c) 1.0 mol L-1 NaHCO3,Diffuse reflectance spectra of ztype photocatalyst composed of (CuGa) 0.5ZNS2 and RGO-(CoOx/BiVO4) before CO2 reduction, (e) after CO2 reduction with 10 mol L-1 NaHCO3 and (f) 100 mol L-1 NaHCO3,(g) Diffuse reflection spectrum of RGO-(CoOx/BiVO4).Figure 7. (a) Cu L3M4,5M4,5 Auger spectra of 0.5ZNS2 before CO2 reduction,Cu L3M4,5M4,5 Auger spectra of 0.5ZnS2 at (B) without NaHCO3 and (c) with 1.0 mol L-1 NaHCO3 after CO2 reduction (CuGa)Z-type photocatalyst composed of (CuGa) 0.5ZNS2 and RGO-(CoOx/BiVO4) before (D)CO2 reduction,(e) Cu L3M4,5M4,5 Auger spectrum, c 1s peak of (a) – (c) and In 3d peak of (d)- (f) after CO2 reduction with the addition of 10 mol L-1 NaHCO3 and (F) 100 mol L-1 NaHCO3 corrected the kinetic energy.Figure 8. (a) ESR of 0.5ZnS2 at 77K before CO2 reduction (CuGa), ESR of 0.5ZnS2 at 77K after CO2 reduction (CuGa) at (b) without NaHCO3 and (c) with 1.0 mol L-1 NaHCO3,A z-type photocatalyst consisting of (CuGa) 0.5ZNS2 and RGO-(CoOx/BiVO4) obtained ESR at 77 K after CO2 reduction in the presence of (D)CO2 reduction, (e) 10 mol L-1 NaHCO3 and (F) 100 mol L-1 NaHCO3.The strength of (b) and (c) is 1/5.FIG. 9 Photochemical CO2 reduction of (CuGa) 0.5ZNS2 photocathode under visible light irradiation in an aqueous solution containing 0.1mol L-1 KHCO3, with a constant bias of 0.1V vs RHE applied.Optical electrode: drop casting, electrolyte: 0.1 mol L-1 KHCO3(AQ) and CO2 dissolved at 1 ATM, light source :300 W Xe lamp (λ > 420 nm), applied bias:0.1V vs RHE(− 0.5V vs Ag/AgCl (pH 6.9)), reference electrode: Ag/AgCl, counter electrode: Pt.In this paper, it is concluded that the (CuGa) 0.5ZNS2 photocatalyst with SO32- as sacrificial electron donor achieves the reduction of CO2 to CO under visible light irradiation.The addition of NaHCO3 to the reactant solution at the expense of CO2 reduction enhanced CO formation, and the selectivity of CO formation reached 42% even in the absence of any cocatalyst.The Z-type photocatalyst composed of bare CuGa0.5ZNS2 and RGO-COOx /BiVO4 achieved the reduction of CO2 to CO by using water as electron donor under visible light irradiation without adding any salt additives.In addition, the addition of basic salts to the reactant solution improves the release of CO, with a CO selectivity of 10-20%, while producing almost stoichiometric O2.The enhancement of z-type CO2 reduction is due to suitable pH conditions and the efficient supply of hydrated CO2 molecules to the reactants by the addition of basic salts.Appropriate concentrations of alkaline additives can also stabilize z-type photocatalysts that use photocorroded metal sulfide materials in CO2 reduction.Therefore, reactant solution adjustment is helpful to improve and stabilize z-type CO2 reduction.In addition to the control of the reactant solution, we successfully enhanced the Z-type CO2 reduction by using an improved (CuGa) 0.5ZNS2 photocatalyst.Under the simulated sunlight, the Z-Scheme system can effectively reduce CO2 to CO by using water as the electron donor.It can be seen from the AES and ESR measurements that z-co2 reduction is more stable than sacrificial CO2 reduction, because the self-photooxidation of Cu+ on (CuGa) 0.5ZNS2 is inhibited during Z-CO2 reduction.The authors also performed photochemical CO2 reduction experiments using the original powder (CuGa)0.5ZnS2 photocatalyst.Typically, PEC systems consist of high quality thin films prepared by complex processes that require surface modification with some thin layer compounds and cocatalysts.In contrast, current PEC using (CuGa) 0.5ZNS2 can achieve reasonable efficiency even when using simple powder materials and without surface modification.The Faraday efficiency of CO formation reaches 21% at 0.1V vs RHE due to the efficient transport of holes to FTO substrate, showing high stability.Therefore, the PEC battery with (CuGa) 0.5ZNS2 photocathode is conducive to effective and stable CO2 reduction.In addition, the (CuGa)0.5ZnS2 photocatalyst itself has excellent electrocatalytic sites for CO2 reduction on the surface.Therefore, by using metal sulfide photocatalyst in a simple water suspension photoelectrochemical system, the photocatalytic reduction of CO2 with high activity, high selectivity and high durability with water as electron donor was achieved under visible light irradiation.The discovery of regulated reactant solution will contribute to the construction of efficient Z-scheme photoelectrochemical system, using metal sulfide photocatalyst to reduce CO2 with water as electron donor under visible light irradiation, accompanied by the production of O2.