Cobalt-to-vanadium charge transfer in polyoxometalate water oxidation catalysts revealed by 2p3d resonant inelastic X-ray scattering

Abstract

Two isostructural cobalt containing polyoxometalate water oxidation catalysts, [Co₄(H₂O)₂(α-PW₉O₃₄)₂]¹⁰⁻ (Co₄P₂) and [Co₄(H₂O)₂(α-VW₉O₃₄)₂]¹⁰⁻ (Co₄V₂), exhibit large differences in their catalytic performance. The substitution of phosphorus centers in Co₄P₂ with redox-active vanadium centers in Co₄V₂ leads to electronic structure modifications. Evidence for the significance of the vanadium centers to catalysis, predicted by theory, was found from soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS). The XAS and RIXS spectra determine the electronic structure of the cobalt and vanadium sites in the pre-reaction state of both Co₄V₂ and Co₄P₂. High-energy resolution RIXS results reveal that Co₄V₂ possesses a smaller ligand field within the tetra-cobalt core and a cobalt-to-vanadium charge transfer band. The differences in electronic structures offer insights into the enhanced catalysis of Co₄V₂.