Understanding the Photocatalytic Reduction of CO2 with Heterometallic Molybdenum(V) Phosphate Polyoxometalates in Aqueous Media

Three crystalline heterometallic molybdenum(V) phosphates have been synthesized under hydrothermal conditions. They all contain {M[P4Mo6O28(OH)(3)](2)}(16-) [M = Mn(II) or Co(II)] polyoxometalate ( POM) units, with the M ions sandwiched between two {P4Mo6V} rings. In the presence of Fe(II) ions in the reaction medium, a three-dimensional (3D) Fe-Mn compound built from the connection of Mn(P4Mo6)(2) units to Fe(II) and Fe(III) centers by extra phosphate ions is obtained. Alternatively, the introduction of [Ru(bpy)(3)](2+) complexes in the synthetic medium prevents the formation of such high-dimensional compounds. In the two Ru(bpy)-Mn and Ru(bpy)-Co hybrids, chains are indeed formed, whereby the Mn(P4Mo6)(2) or Co(P4Mo6)(2) anions are bridged by Mn(II) or Co(II) ions, respectively. The charge of these anionic chains is compensated by neighboring [Ru(bpy)(3)](2+) complexes. Among these three compounds, only Fe-Mn and Ru(bpy)-Mn are active for the heterogeneous photocatalytic reduction of CO2 into CH4 as the major product and CO (yield in CH4 of 1440 and 600 nmol g(-1) h(-1) with selectivity in CH4 equal to 92.6 and 85.2%, respectively, under 8 h irradiation) in water, in the presence of triethanolamine (TEOA) as an electron donor and [Ru(bpy)(3)](2+) as a photosensitizer. A density functional theory (DFT) analysis allowed for proposing a reaction mechanism involving the formation of a solvated electron via photoionization of a one-electron reduced [Ru-II(bpy)(2)(bpy(center dot-))](+) complex as the key step to reduce CO2 to CO2(center dot-). The latter can then coordinate to the peripheral M(II) ions to yield CO through electron- and proton-transfer steps involving reduced POMs and protons generated in the photooxidation of the sacrificial donor. Concerning the nonactive compound, Ru(bpy)-Co, DFT calculations revealed that the Co(II) dimers present in the structure may spontaneously take the extra electron out of CO2(center dot-) to form a Co-Co bond, releasing CO2 back. Finally, preliminary results suggest that the reduction of CO to CH4 could be photochemically accomplished by the POM-based materials in the presence of TEOA, with no mechanistic requirement for the participation of [Ru(bpy)(3)](2+).