Visible-light CO2 photoreduction of polyoxometalate-based hybrids with different cobalt clusters

Abstract

The photoreduction of carbon dioxide (CO₂) into a valuable energy gas (CO and H₂) is an efficient approach to address the fossil fuel crisis and mitigate the global warming effect. The development of effective photocatalysts for CO₂ reduction is still desirable and challenging. Herein, two novel polyoxometalate-based hybrids with multinuclear cobalt clusters, [Co_2.67(SiW₁₂O₄₀)(H₂O)₄(Htrz)₄]·Cl_1.33 (Htrz = 1,2,4-triazole) (1) with a binuclear cobalt cluster and [Co₃(SiW₁₂O₄₀)(H₂O)₃(Htrz)₆Cl]·Cl·6H₂O (2) with a trinuclear cobalt cluster, were synthesized under hydrothermal conditions. Both of them were characterized by single-crystal X-ray diffractions, PXRD, IR, TG and UV–vis spectra. Compound 1 exhibited a 3D structure with 4-connected [SiW₁₂O₄₀] and 3-connected [Co₂(Htrz)₃(H₂O)₃] secondary building units (SBUs). The 1D chain of compound 2 was constructed from [SiW₁₂O₄₀] and [Co₃(Htrz)₆(H₂O)₃Cl] SBUs. Furthermore, the photoreduction of CO₂ under visible light by the two cobalt-based POMs was investigated using [Ru(bpy)₃]Cl₂·6H₂O as a photosensitizer. The CO yields of compounds 1 and 2 were 15 705 and 18 501 μmol g⁻¹ for the CO₂ photocatalytic reduction under three hour irradiation at 293 K, respectively. The difference in the photocatalytic performance of 1 and 2 was explained by comparing the energy of the valence band, band gaps and conduction band. The results showed that the photocatalysts incorporated with multinuclear Co clusters could effectively improve photocatalytic activities, thus providing a valuable view to design high performance and cost-acceptable molecular catalysts for CO₂ photoreduction.