While two-dimensional (2D) undoped layered oxynitrides are potential candidates as high performance visible-light photocatalysts, synthesis of 2D layered oxynitrides is generally difficult because most of them are metastable phases and are not chemically very stable. We found an exceptional example of a 2D layered oxynitride, K₂LaTa₂O₆N (more specifically, K₂LaTa₂O₆N·1.6H₂O), which is a Ruddlesden-Popper phase two-layer perovskite analogous to Li₂LaTa₂O₆N and has a band gap of 2.5 eV. This material undergoes in situH⁺/K⁺ exchange in aqueous solution while keeping its visible light absorption capability. The protonated, Ir-modified K₂LaTa₂O₆N exhibited photocatalytic activity for H₂ evolution from aqueous NaI solution under visible light, outperforming Pt/ZrO₂/TaON and Pt/SrTiO₃:Rh, which are one of the best-performing oxynitride and oxide photocatalysts for H₂ evolution, respectively.

The use of mixed-anion materials is of interest not only for photocatalysts but also for catalysts promoting water oxidation and CO₂reduction. For example, we found that certain oxyfluorides work as electrocatalysts for water oxidation. A 2D perovskite oxyfluoride Pb₃Fe₂O₅F₂ exhibits activity for electrochemical water oxidation to O₂, which is 8 times more active than a 3D bulk PbFeO₂F.