Photocatalytic Conversion of CO2 By H2o As an Electron Donor over Mg-Doped SrTiO3
Authors: Nakamoto Takechi , Iguchi Shoji , Naniwa Shimpei , Tanaka Tsunehiro , Teramura Kentaro 
Published: September 2023

Atmospheric CO2 is considered as the main cause of climate change, which is known as one of the serious problems for the earth, and several approaches are employed to convert CO2 into value-added products, such as CO, HCOOH, CH3OH and CH4. Among those approaches, photocatalysis has been highly attracting the world-wide attention, because photocatalytic conversion of CO2, which is called as “artificial photosynthesis”, can be a solution for climate change in terms of its renewability, harmlessness and low cost. Ga-based mixed oxide photocatalysts such as ZnGa2O4 were reported as superior photocatalysts for the photocatalytic conversion of CO2 by using H2O3. Recently, our group reported that Ag and Co dual cocatalysts-loaded Al-doped SrTiO3 photocatalyst showed relatively high formation rate of CO for the photocatalytic conversion of CO2 by H2O under photoirradiation with a monochromatic light (365 nm)1, although Ga-based photocatalysts exhibited the activity under the photoirradiation with less than 300 nm. In this study, the effect of metal dopant in SrTiO3 on the photocatalytic conversion of CO2 was investigated. It was found that Mg-doped SrTiO3 showed higher formation rate of CO for the photocatalytic conversion of CO2 into CO under photoirradiation at 365 nm than Al-doped SrTiO3.

Mg-doped SrTiO3 was synthesized by a flux method. After grinding mixture of SrTiO3, MgO, and SrCl2 flux, the obtained powder was calcined at 1418 K for 15 h. Ag and Co dual cocatalysts were loaded by chemical reduction method. An aqueous solution of NaH2PO2 reductant was added to the suspension containing Mg-doped SrTiO3, AgNO3, and Co(NO3)2, and it was kept at 353 K for 1.5 h with stirring. The photocatalytic conversion of CO2 by H2O was carried out using an external-irradiation-type reaction vessel. Ag and Co-loaded Mg-doped SrTiO3 was dispersed in 0.1 M NaHCO3 aqueous solution, and high purity CO2 gas was bubbled into the suspension at a flow rate of 30 mL min−1. 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 methanizer (CO).

XRD patterns of pristine and Mg-doped SrTiO3 showed that diffraction peaks are shifted to the lower angle by Mg2+ doping, which might be corresponded to Mg2+ doping into Ti4+ site in the bulk of SrTiO3. SEM images displayed that a shape of Mg-doped SrTiO3 is an edge-shaved cube, while that of pristine SrTiO3 is irregular. In other words, {110} facets were observed in Mg-doped SrTiO3 in addition to {100} facets. According to the previous report2, the photoexcited carriers, electrons and holes, were transported to different crystal facets of the perovskite oxide, especially {100} and {110} facets, respectively. This anisotropic charge distribution would decrease the charge recombination rate in Mg-doped SrTiO3, resulting in the high photocatalytic activity for CO2 conversion.

The attached figure shows formation rates of H2 (gray), O2 (white) and CO (black) and selectivity toward CO evolution (black diamond) for the photocatalytic conversion of CO2 by H2O over Ag and Co-loaded pristine SrTiO3, Al-doped SrTiO3, and Mg-doped SrTiO3. Mg-doped SrTiO3 exhibited much higher photocatalytic activity than pristine SrTiO3 and Al-doped SrTiO3. The formation rate of CO over Mg-doped SrTiO3 was 20 μmol h−1, which was 2.3 times larger than that over Al-doped SrTiO3. Surprisingly, the selectivity toward CO evolution was almost 100 % over both Al-doped and Mg-doped SrTiO3. Moreover, the stoichiometric amount of O2 evolution was observed during this reaction, indicating that H2O worked as an electron donor for the photocatalytic conversion of CO2.

The control experiments revealed that the reduction of CO2 did not proceed in the absence of photocatalyst, photoirradiation, or additive (0.1 M NaHCO3). These results clearly indicated that the significant photocatalytic performance was observed by using the Ag and Co-loaded Mg-doped SrTiO3 photocatalyst under photoirradiation with a NaHCO3 additive in the suspension. To confirm that the generated CO is not derived from any organic contamination, the isotope-labeling experiments using 13CO2 gas was carried out. GC-MS analysis of the outlet gas in the photocatalytic conversion of 13CO2 over Mg-doped SrTiO3 clarified that 13C-labeled CO (m/z = 29) was evolved in priority to 12CO (m/z = 28). Accordingly, we concluded that CO gas evolved in our system is originated from CO2 gas bubbled into the suspension.

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Figure 1