Porphyrin-Sensitized Evolution of Hydrogen using Dawson and Keplerate Polyoxometalate Photocatalysts

Hydrogen evolution using photocatalytic systems based on artificial photosynthesis is a major approach toward solar energy conversion and storage. In the polyoxometalate-based photocatalytic systems proposed in the past, middle/near UV light irradiation and noble-metal catalysts were mainly used. Although recently polyoxometalates were sensitized in visible light, photosensitizers or catalysts based on noble metals, and/or poor activity of polyoxometalates were generally obtained. Here we show the highly efficient [turnover number (TON)=215] hydrogen evolution induced by the zinc(II) mesotetrakis(N-methyl-pyridinium-4-yl)porphyrin (ZnTMPyP⁴⁺ ) sensitization of a series of polyoxometalate catalysts (two Dawson type, P₂ Mo₁₈ O₆₂^6- and P₂ W₁₈ O₆₂^6- anions, and one Keplerate {Mo₁₃₂ } cluster) in a visible-light-driven, noble-metal-free, and fully water-soluble system. We attributed the high efficiency for hydrogen evolution to the multi-electron reduction of polyoxometalates and found that: (a) both Dawson polyoxometalates exhibit higher hydrogen evolution efficiency upon ZnTMPyP⁴⁺ sensitization in relation to the direct photoreduction of those compounds; (b) the P₂ Mo₁₈ O₆₂^6- anion is more efficient (TON=65 vs. 38, respectively) for hydrogen evolution than the P₂ W₁₈ O₆₂^6- anion; and (c) the high nuclearity Keplerate {Mo₁₃₂ } cluster exhibits the highest efficiency (TON=215) for hydrogen evolution compared with the polyoxometalates studied.