Development of functional polyoxometalate-based catalysts for symbioses with nature

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Molecular hydrogen is known to be a clean-burning fuel free of CO_2 emissions; it is considered a promising candidate to mitigate the current energy problems. The development of efficient photocatalysts for hydrogen production from water has attracted much attention in the fields of solar light energy utilization and storage. In this work, we synthesized Dawson- and Keggin-type dirhenium(V)-containing polyoxometalates [O{Re^V(OH)(α_2-P_2W_<17>O_<61>)}_2]^<14-> (1) and [PW_<11>Re^VO_<40>]^<4-> (2). The potassium salt of 1 K_<14>[O{Re^V(OH)(α_2-P_2W_<17>O_<61>)}_2]・21H_2O (K-1) and the dimethylammonium salt of 2 [Me_2NH_2]_4[PW_<11>Re^VO_<40>] (Me_2NH_2-2) were obtained as analytically pure homogeneous black-purple crystals by reacting mono-lacunary Dawson- and Keggin-type polyoxotungstate with [Re^<IV>Cl_6]^<2-> in water, followed by crystallization from HCl-acidic solution and acetone, respectively. Furthermore, K-1 and Me_2NH_2-2 were supported onto anatase TiO_2 surface by the precipitation methods using CsCl and Pt(NH_3)_4Cl_2. With these materials, hydrogen evolution from water in the presence of EDTA・2Na (ethylenediamine tetraacetic acid disodium salt) under visible light irradiation (〓400nm) was achieved.