Metal Doping in SrTiO3 Photocatalyst to Improve the Photocatalytic Activity for CO2 Conversion

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 oxides, titanium-based oxides, gallium-based oxides, and niobium-based oxides 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. As continuous efforts to develop effective photocatalysts for the photocatalytic conversion of CO2 to CO in water, in this study, we investigated metal doping in the SrTiO3 photocatalyst, and evaluated the photocatalytic activity of the doped SrTiO3 photocatalysts for the conversion of CO2 to CO under UV-LED-light irradiation at 365 nm. SrTiO3 photocatalyst was prepared by a previously reported solid-state method (Journal of Power Sources, 2008, 183, 701−707). SrCO3 and TiO2 were ground for 15 min in a mortar and pestle, and the mixture was transferred to an alumina crucible to calcine at 1373 K for 10 h in air. The resulting powder was washed three times with hot ultrapure water and dried overnight at room temperature. M-doped SrTiO3 (M−SrTiO3, M = Al, Zn, Li, Mn, W, Ca, Y, and Mg) was synthesized via a flux method (ACS Applied Energy Materials, 2020, 3, 1468−1475.). Excess amount of SrCl2 flux was added to a mixture of the prepared SrTiO3 and a metal oxide as a doping source. The mixture was ground for 15 min in a mortar and pestle, transferred to an yttria crucible, and calcined at 1418 K for 15 h. The obtained powder was washed three times with hot ultrapure water, and dried overnight at 353 K. Photocatalytic activity of AgCo/M−SrTiO3 photocatalyst was evaluated by using an external-irradiation type reaction vessel in a quasi-flowing batch system. The photocatalyst powder was dispersed in 0.2 L of an aqueous NaHCO3 solution, and the dissolved air in the suspension was degassed by a flow of high-purity CO2 gas. CO2 gas was continuously bubbled into the reaction solution at a flow rate of 30 mL/min. The suspension was irradiated using a monochromatic UV-LED lamp at 365 nm (IRS-1000, CELL System Co., Ltd., Japan). 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 the M−SrTiO3 photocatalysts (M = Al, Zn, Li, Mn, W, Ca, Y, and Mg) in the presence of AgCo cocatalyst. Zn−, Li−, Mn−, W−, Ca−, and Y−SrTiO3 showed extremely low photocatalytic activities for the conversion of CO2, whereas Al− and Mg−SrTiO3 produced certain amount of CO and O2. Mg−SrTiO3 exhibited the highest activity for CO2 conversion, which has been observed as a CO formation rate of ca. 20 μmol h−1. However, H2 evolution was thoroughly suppressed (0.055 μmol h−1) in the presence of AgCo/Mg−SrTiO3, where the selectivity toward CO evolution was higher than 99%. These results indicated that Mg-doping into SrTiO3 improved the photocatalytic activity for the conversion of CO2. Moreover, stoichiometric O2 evolution (8.9 μmol h−1) was found in the presence of AgCo/Mg−SrTiO3, suggesting that H2O acts as both an electron donor and a proton source for the photocatalytic conversion of CO2 in water. The apparent quantum efficiency (AQE) of AgCo/Mg−SrTiO3 photocatalyst in the photocatalytic conversion of CO2 under monochromatic UV-light irradiation (365 nm) was determined to be 0.05%. Figure 1