Photoreduction of carbon dioxide and photodegradation of organic pollutants using alkali cobalt oxides MCoO₂ (M = Li or Na) as catalysts

The recycling of lithium batteries should be prioritized, and the use of discarded alkali metal battery electrode materials as photocatalysts merits research attention. This study synthesized alkali metal cobalt oxide (MCoO₂, M = Li or Na) as a photocatalyst for the photoreduction of CO₂ and degradation of toxic organic substances. The optimized NaCoO₂ and LiCoO₂ photocatalysts increased the photocatalytic CO₂-CH₄ conversion rate to 21.0 and 13.4 μmol g^-1 h^-1 under ultraviolet light irradiation and to 16.2 and 5.3 μmol g^-1 h^-1 under visible light irradiation, which is 17 times higher than that achieved by TiO₂ P25. The rate constants of the optimized reactions of crystal violet (CV) with LiCoO₂ and NaCoO₂ were 2.29 × 10^-2 and 4.35 × 10^-2 h^-1, respectively. The quenching effect of the scavengers and electron paramagnetic resonance in CV degradation indicated that active O₂^•-, ¹O₂, and h⁺ play the main role, whereas ^•OH plays a minor role for LiCoO₂. The hyperfine splitting of the DMPO-^•OH and DMPO-^•CH₃ adducts was a_N = 1.508 mT, a_Hβ = 1.478 mT and a_N = 1.558 mT, a_Hβ = 2.267 mT, respectively, whereas the hyperfine splitting of DMPO^+• was a_N = 1.475 mT. The quenching effect also indicated that active O₂^•- and h⁺ play the main role and that ^•OH and ¹O₂ play a minor role for NaCoO₂. The hyperfine splitting of the DMPO-^•OH and DMPO^+• adducts was a_N = 1.517 mT, a_Hβ = 1.489 mT and a_N = 1.496 mT, respectively. Discarded alkali metal battery electrode materials can be reused as photocatalysts to address environmental pollution.