Extending the visible-light photocatalytic CO2 reduction activity of K2Ti6O13 with the MxOy (M = Co, Ni and Cu) incorporation

Co3O4, NiO, and CuO-K2Ti6O13 composites were successfully grown in-situ by using an ultrasound-assisted sol–gel method. As a result of the synthesis method, the introduction of the metallic cations into the crystalline structure substituting Ti4+ or K+ cations depending on the case was achieved. For instance, Co2+ cations were introduced instead of Ti4+ cations, while Ni2+ and Cu2+ were introduced on both sides. The metal-cation introduction between the tunnels favored the growth of K-poor phases as impurities, especially in the CuO loaded K2Ti6O13 samples. The presence of these metallic oxides modified the structural and optical properties by forming oxygen vacancies in some samples, favoring the photocatalytic CO2 reduction in aqueous media to low-weight compounds, such as formaldehyde, methanol, methane, and hydrogen, under visible-light irradiation. Enhanced selectivity of the evolved product, as a result of the metallic cation nature and the formed impurities, was observed. For instance, Co3O4 favored the evolution of formaldehyde in the most efficient sample (1 Co-KTO; 453.2 μmol g−1) as a result of the low quantity of impurities present in samples; NiO favored the hydrogen evolution reaction (1 Ni-KTO; 201 μmol g−1), possibly due to the photo-reforming of the organic compounds; and CuO enhanced the methanol and hydrogen production (159.7 and 282 μmol g−1, respectively in 1 Cu-KTO) due to the formation of oxygen vacancies as active sites for the photocatalytic process and reducing the photo-generated charge recombination.