Nanostructured Alkaline‐Cation‐Containing δ‐MnO₂ for Photocatalytic Water Oxidation

Abstract Oxygen evolution from water is one of the key reactions for solar fuel production. Here, two nanostructured K‐containing δ‐MnO₂ are synthesized: K‐δ‐MnO₂ nanosheets and K‐δ‐MnO₂ nanoparticles, both of which exhibit high catalytic activity in visible‐light‐driven water oxidation. The role of alkaline cations in oxygen evolution is first explored by replacing the K⁺ ions in the δ‐MnO₂ structure with H⁺ ions through proton ion exchange. H‐δ‐MnO₂ catalysts with a similar morphology and crystal structure exhibit activities per surface site approximately one order of magnitude lower than that of K‐δ‐MnO₂, although both nanostructured H‐δ‐MnO₂ catalysts have much larger Brunauer–Emmett–Teller (BET) surface areas. Such a low turnover frequency (TOF) per surface Mn atom might be due to the fact that the Ru²⁺(bpy)₃ sensitizer is too large to access the additional surface area created during proton exchange. Also, a prepared Na‐containing δ‐MnO₂ material with an identical crystal structure exhibits a TOF similar to that of the K‐containing δ‐MnO₂, suggesting that the alkaline cations are not directly involved in catalytic water oxidation, but instead stabilize the layered structure of the δ‐MnO₂.