Understanding the Photocatalytic Reduction of CO2with 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 {P4MoV6} 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 [RuII(bpy)2(bpy•–)]+ complex as the key step to reduce CO2 to CO2•–. 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•– 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+.