(Invited) Photocatalysis of Mg-Doped SrTiO3 for Conversion of CO2 with H2O
Authors: Iguchi Shoji , Nakamoto Takechi , Naniwa Shimpei , Tanaka Tsunehiro , Teramura Kentaro 
Published: August 2024

Photocatalysis has been highly attracting the world-wide attentions among many approaches to convert CO2 into value-added products such as CO, HCOOH, CH3OH and CH4, because photocatalytic conversion of CO2 can be a solution for global warming in terms of its renewability, harmlessness, relatively low cost, and stability. Large number of photocatalysts reported in the research field of the photocatalytic conversion of CO2 require the photoirradiation with deep UV light < 300 nm. Recently, our research group found that Al-doped SrTiO3 photocatalyst (Al-STO), which has been previously reported as a superior photocatalyst for overall water splitting[1], also showed the photocatalytic activity for the conversion of CO2 into CO[2]. In this study, the effect of metal dopant in SrTiO3 on the photocatalytic conversion of CO2 was investigated, and it was found that Mg-doped SrTiO3 showed excellent photocatalytic activity for the conversion of CO2 into CO in the presence of AgCo dual cocatalyst[3].

Mg-doped SrTiO3 (Mg-STO) was synthesized by a flux method; the mixture of SrTiO3, MgO, and SrCl2 flux was calcined at 1418 K for 15 h. Ag and Co dual cocatalyst was loaded by a chemical reduction method. An aqueous solution of NaH2PO2 reductant was added to the suspension containing Mg-STO, AgNO3, and Co(NO3)2, and it was kept at 353 K for 90 min with vigorous stirring. Photocatalytic conversion of CO2 with H2O was carried out using an external-irradiation-type reaction vessel. AgCo/Mg-STO was dispersed in an aqueous solution of 0.1 M NaHCO3, and high purity CO2 gas was bubbled into the suspension at a flow rate of 30 mL min−1. The monochromatic UV LED lamp (365 nm) was used as a light source. The gaseous products were analyzed by TCD-GC (H2, O2) and FID-GC equipped with a methanizer (CO).

AgCo/Mg-STO exhibited much higher photocatalytic activity for the conversion of CO2 into CO than AgCo/STO and AgCo/Al-STO, which were previously reported. CO was produced at a formation rate of 20 μmol h−1, which was 2.3 times larger than that for Al-STO, and the selectivity toward CO evolution was almost 100 %. H2 formation was completely suppressed in this case. Moreover, the stoichiometric amount of O2 was evolved, indicating that H2O should contribute to the photocatalytic conversion of CO2 as an electron donor. The isotope-labeling experiments using 13CO2 gas as a substrate revealed that the carbon source of CO evolved in this photocatalytic reaction CO2 gas bubbled into the suspension. Therefore, we concluded that CO2 surely split into CO and O2 by the photocatalysis of AgCo/Mg-STO. According to SEM images of undoped STO and Mg-STO, particle shape was obviously changed by Mg doping from pure cubes to irregular edge-shaved cubes. {110} facets were observed in Mg-STO accompanied with {100} facets. Moreover, Ag and Co species were selectively located on the {100} facets and the {110} edge-shaved facets, respectively, after the photoirradiation. The photogenerated electrons and holes were considered to be transferred to {100} and {110} facets separately, and the anisotropic charge transfer should result in the high photocatalytic activity of Mg-STO for the conversion of CO2 with H2O.

[1] T. Takata et al., Nature, 2020, 581, 411-414.

[2] S. Wang et al., Chem. Sci., 2021, 12, 4940-4948.

[3] T. Nakamoto et al., Catal. Sci. Technol., 2023, 13, 4534-4541.