Two‐Dimensional Perovskite Oxynitride K 2 LaTa 2 O 6 N with an H + /K + Exchangeability in Aqueous Solution Forming a Stable Photocatalyst for Visible‐Light H 2 Evolution

Undoped layered oxynitrides have not been considered as promising H2-evolution photocatalysts because of the low chemical stability of oxynitrides in aqueous solution. Here, we demonstrate the synthesis of a new layered perovskite oxynitride, K2LaTa2O6N, as an exceptional example of a water-tolerant photocatalyst for H2 evolution under visible light. The material underwent in-situ H+/K+ exchange in aqueous solution while keeping its visible-light-absorption capability. Protonated K2LaTa2O6N, modified with an Ir cocatalyst, exhibited excellent catalytic activity toward H2 evolution in the presence of I− as an electron donor and under visible light; the activity was six times higher than Pt/ZrO2/TaON, one of the best-performing oxynitride photocatalysts for H2 evolution. Overall water splitting was also achieved using the Ir-loaded, protonated K2LaTa2O6N in combination with Cs-modified Pt/WO3 as an O2 evolution photocatalyst in the presence of an I3−/I− shuttle redox couple.