Developing Insoluble Polyoxometalate Clusters to Bridge Homogeneous and Heterogeneous Water Oxidation Photocatalysis

Abstract

Cluster catalysts are attractive for their atomically precise structures, defined compositions, tunable coordination environments, uniform active sites, and their ability to transfer multiple electrons, but they suffer from poor stability and recyclability. Here, we report a general approach to the direct insolubilization of a water soluble polyoxometalate (POM) [{(B-α-PW₉ O₃₄ )Co₃ (OH)(H₂ O)₂ (O₃ PC(O)-(C₃ H₆ NH₃ )PO₃ )}₂ Co]^14- (Co₇ ) and formation of a series of POM-based solid catalysts with the counter-cations Ag⁺ , Cs⁺ , Sr²⁺ , Ba²⁺ , Pb²⁺ , Y³⁺ , and Ce³⁺ . They exhibit improved catalytic activities for visible-light-driven water oxidation following the trend CsCo₇ >SrCo₇ >AgCo₇ >Ce^III Co₇ >BaCo₇ >YCo₇ >PbCo₇ . While CsCo₇ exhibits mainly homogeneous catalysis, the others are predominantly heterogeneous catalysts. An optimal oxygen yield of 41.3 % and a high apparent quantum yield (AQY) of 30.6 % for SrCo₇ is obtained, which is comparable to that of the parent homogeneous POM. Band gap structures, UV/Vis spectra, and real-time laser flash photolysis experiments collectively suggest that easier electron transfer from the solid POM catalyst to the photosensitizer promotes photocatalytic water oxidation performance. These solid POM catalysts exhibit good stability, which is directly confirmed by a combination of Fourier-transform infrared spectroscopy, electron microscopy, X-ray diffraction patterns, Raman spectroscopy, X-ray photoelectron spectroscopy, five cycles of tests, and poisoning experiments.