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: [Cu^II₅(2-ptz)₆(H₂O)₄(GeW₁₂O₄₀)]·4H₂O (1) and [Cu^I₂(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 [Cu^II₅(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.