Improving the photocatalytic H2 evolution activity of Keggin polyoxometalates anchoring copper-azole complexes†
Abstract
Eliminating the use of precious metals as cocatalysts and using visible light are two important aspects in the field of photocatalytic H2 evolution with polyoxometalates (POMs) as photosensitizers. Here we present two new POM-based materials: [CuII5(2-ptz)6(H2O)4(GeW12O40)]·4H2O (1) and [CuI2(ppz)4][H2GeW12O40]·8H2O (2) (2-ptz = 5-(2-pyridyl) tetrazole, ppz = 3-(pyrid-4-yl) pyrazole) synthesized with the Keggin type [GeW12O40]4− (GeW12) polyanion and copper-azole complexes. The optimum photocatalytic H2 evolution rate of compound 1 without a noble metal cocatalyst is 3813 μmol g−1 h−1, which is 7.6 times higher than that of compound 2 and more than 27 times higher than that of bare GeW12 polyanions. The improved photocatalytic H2 evolution rate of compound 1 is mainly attributed to: (i) the formation of stable heteropoly blue species that can absorb visible light, (ii) the large differences in the reduction potentials of the GeW12 polyanion and the copper complexes that inhibit photogenerated electron–hole pair recombination and (iii) the good dispersion of the GeW12 polyanions, thanks to the presence of [CuII5(2-ptz)6(H2O)4]n4n+ layers that preclude their aggregation. To the best of our knowledge, compound 1 is the photosensitizer for H2 evolution with the highest photocatalytic performance of all the classical POM-based materials.
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