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Proton-exchanged K4N b6OI7 showed high activity for H2 evolution from aqueous methanol solution without any assistance from other materials such as noble metals.Photocatalytic reactions over semiconductors such as TiOz, CdS, and SrTi03 have been studied by many workers.' Recently we have reported that K4Nb6OI7 photocatalyst exhibits high activity for H2 evolution from aqueous methanol solution and that a pretreated NiO (0.1 wt%)-K4NbhO17 photocatalyst can decompose H 2 0 into H2 and 0 2 stoicheiometrically.2 K4Nb6OI7 has a layer structure ,3 and separating the layers are two types of interlayers. One type of interlayer contains water molecules and potassium ions and the other contains only potassium ions. The potassium ions between niobium oxide layers can be replaced with many other cations including transition metal ions.4 We report here that among ion-exchanged K4NbhOI7 photocatalysts, H+/K4Nb6OI7 shows the highest photocatalytic activity for H2 evolution from aqueous methanol solution. As far as we know, this is the first example of a metal oxide which shows high and stable activity for H2 evolution without any assistance from other materials such as noble metals, although ZnS is also known to have a similar high activity for H2 e v~l u t i o n .~ K4Nb60,, was prepared by calcination of a mixture of