(Invited) Al-Doped SrTiO3 Photocatalyst for Selective CO2 Reduction with Water as a Reductant

Abstract

Among various strategies toward finding new energy sources to replace conventional fossil fuels, photocatalytic conversion of CO2 into useful chemicals using water as an electron donor and a proton source has been increasingly attracting attentions because it can convert light energy to chemical energy. The photocatalytic conversion of CO2 involves photoabsorption (light harvesting), charge separation, and H2O oxidation, as well as the consumption of the photogenerated electrons for reduction. The evolution of H2 rather than CO is preferred when H2O is used as the electron donor, since the redox potential of CO2/CO (−0.51 V vs NHE, at pH 7) is more negative than that of H+/H2 (−0.41 V vs NHE, at pH 7) in an aqueous solution. Therefore, “high selectivity toward CO2 reduction” is one of the most important requirements for the photocatalytic system to achieve the CO2 conversion in water. Various mixed oxides including tantalum-based NaTaO3, Ta2O5, Sr2KTa5O15, titanium-based BaLa4Ti4O15, CaTiO3, Na2Ti3O7, K2Ti6O13, La2Ti2O7, SrTiO3, gallium-based Ga2O3, and ZnGa2O4, and SrNb2O6 were found to be excellent photocatalysts for the conversion of CO2 into CO in the presence of Ag cocatalyst. However, these materials show significant activity for CO evolution only when irradiated with deep region UV-light (λ < 300 nm). Therefore, a series of photocatalysts which can drive under the photoirradiation with UV-light at λ > 300 nm is strongly desired in the research area of the photocatalytic conversion of CO2. Recently, Domen et al. reported that Al-doped SrTiO3 (Al-STO) with suitable cocatalysts such as MoOy/RhCrOx exhibited a promising photocatalytic activity for overall water splitting under photoirradiation at λ > 300 nm; the apparent quantum efficiency of the MoOy/RhCrOx/Al-STO reached close to 69% at 365 nm. Thus, we herein describe our investigation into the photocatalytic conversion of CO2 by H2O using the Al-STO photocatalyst with Ag and AgCo cocatalysts under photoirradiation with λ > 300 nm. Al-STO photocatalyst was fabricated by a flux method using anhydrous SrCl2 as a flux reagent. A mixture of SrTiO3, Al2O3, and anhydrous SrCl2 was ground for 10 min, and then transferred into an alumina crucible and calcined at 1423 K under air for 15 h. Ag cocatalyst was employed to modify the surface of the as-prepared Al-STO photocatalyst via a chemical reduction method. Al-STO was dispersed in deionized ultrapure water, and AgNO3 and NaH2PO2 were added to the suspension, then it was maintained at 353 K for 1.5 h. Photocatalytic activity of Ag/Al-STO photocatalyst was evaluated by using an inner-irradiation type reaction vessel in a quasi-flowing batch system. The photocatalyst powder was dispersed in 1.0 L of an aqueous NaHCO3 solution, and the dissolved air in this suspension was degassed by a flow of high-purity CO2 gas. CO2 was continuously bubbled into the reaction solution at a flow rate of 30 mL/min. The suspension was irradiated using a 400 W high-pressure Hg lamp with a Pyrex filter equipped with a cooling water system (irradiation wavelength: λ > 300 nm). The gaseous products in the outlet gas, e.g., CO, H2, and O2, were analyzed by gas chromatography (FID-GC with methanizer: CO, TCD-GC: H2 and O2). The attached figure shows a formation rate of CO (red), H2 (blue), and O2 (green) in the photocatalytic conversion of CO2 with H2O using undoped STO and Al-STO in the presence of Ag cocatalyst. It should be noted that the formation rate of CO is significantly increased by the doping of Al into STO. Moreover, the addition of Co species as a second cocatalyst to Ag/Al-STO drastically improved its activity for the photocatalytic conversion of CO2 by H2O as the electron donor, with extremely high selectivity toward CO evolution (99.8%), in which Ag and Co might enable CO2 reduction and H2O oxidation on the Al-STO surface, respectively. The formation rate of CO up to 52.7 μmol/h was observed over AgCo/Al-STO when irradiated with the UV light at wavelengths above 300 nm, which is ten-times higher than that over Ag/Al-STO (4.7 μmol/h). Furthermore, isotope-experiments using 13C-labeled CO2 gas revealed that 13C-labeled CO (m/z = 29) is selectively observed by GC-MS analysis, indicating that the CO evolved over the Ag/Al-STO and AgCo/Al-STO photocatalysts should be derived from the introduced CO2 gas. Figure caption Formation rates of products and selectivity toward CO evolution in the photocatalytic conversion of CO2 using bare Ag/STO (undoped STO), Ag/ and AgCo/Al-STO. Red: CO, green: O2, blue: H2, black circle: selectivity. Loading amount of Ag: 1wt%, Ag : Co = 2 : 1 (mol). Figure 1