Reduced polyoxometalates and bipyridine ruthenium complex forming a tunable photocatalytic system for high efficient CO2 reduction

Photocatalytic CO2 reduction (CO2RR) into value-added chemicals is a promising approach to address global warming and develop sustainable energy, but the key challenge is seeking efficient photocatalytic systems with high performance and straightforward mechanism as well. Herein, we present three new reduced polyoxometalate-based hybrid compounds, Na-6[Co(H2O)(2)(H(2)tib)](2){Co[Mo6O15(HPO4)(4)](2)}center dot 5H(2)O (1), Na-3[Co(H2O)(3)][Co-2(bib)] (H(2)bib)(2.5){HCo [Mo6O14(OH)(HPO4)(4)](2)}center dot 4H(2)O (2) and (H(2)bpp)(3) {HNa[Mo6O12(OH)(3)(HPO4)(3)(H2PO4)](2)}center dot 6H(2)O (3) (bib = 1,4-bis(1-H-imidazol-4-yl)benzene, tib = 1,3,5-tris(1-imidazolyl)benzene, bpp = 1,3-bi(4-pyridyl)propane), which can combine with [Ru(bpy)(3)]Cl-2 complex to form a new tunable photocatalytic system (abbr. {P4Mo6}/Ru(bpy)) for CO2RR. 1/Ru(bpy) and 2/Ru(bpy) exhibit promising photocatalytic CO2RR performance into CO with the yield of 322.7 and 281.81 nmol and the selectivity of 96.3% and 96.4%, respectively. Conversely, 3/Ru(bpy) shows poor photocatalytic CO2RR activity. The photocatalytic mechanism of the {P4Mo6}/Ru(bpy) photocatalytic system was investigated by a series of control experiments and transient photovoltage measurements, which suggested that {P4Mo6}-based POMs act as the sites of oxidizing TEOA and Ru(bpy) served as the center of reducing CO2. The modified cobalt cation on {P4Mo6} polyoxoanions can optimize the energy band structure of parent {P4Mo6}, which regulates the electron transfer pathway of the {P4Mo6}/Ru(bpy) photocatalytic systems, thus achieving highly efficient photocatalytic CO2RR. This work shows a specific mechanism of photocatalytic reduction of CO2 and provides a guidance for the design of new tunable photocatalytic systems for high efficient CO2RR.