Understanding the Photocatalytic Reduction of CO₂with Heterometallic Molybdenum(V) Phosphate Polyoxometalates in Aqueous Media

Abstract

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 {P4Mo V 6} rings. In the presence of Fe(II) ions in the reaction medium, a 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 act as heterogeneous catalysts for the photocatalytic reduction of CO2 into CH4 as the major product and CO (yield in CH4 of 1440 nmol g -1 h -1 and 600 nmol g -1 h -1 with a selectivity in CH4 equal to 92.6 and 85.2%, respectively, under 8 h irradiation) in water, in the presence of TEOA as an electron donor and [Ru(bpy)3] 2+ as a photosensitizer. A DFT analysis allowed proposing a reaction mechanism involving the formation of a solvated electron via photoionization of a one-electron reduced [Ru II (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 non-active 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+ .

Co‐Doped Mn3O4 Nanocubes via Galvanic Replacement Reactions for Photocatalytic Reduction of CO2 with High Turnover Number

“…Upon excitation of reaction system with 500 nm light, a characteristic emission band at around 605 nm of the photosensitizer is observed. The band intensity decreases dramatically with increasing TEOA content in the mixture of MeCN and H 2 O (Figure 4c), which indicates TEOA is an efficient quencher for excited states of [Ru(bpy) 3 ] 2 + * , [37] and the recombination of excited charge carriers is substantially suppressed resulting in the decreased emission band intensity. [38] The band intensity has negligible decrease with increasing the Co x Mn 3-x O 4 content in the reaction system (Figure S9), which can be attributed to a reductive quenching instead of an oxidative one for excited states of [Ru(bpy It is worth noting that the color of Co x Mn 3-x O 4 is changed from brown to light gray (Figure S10) after photocatalytic CO 2 reduction reaction.…”

“…Upon excitation of reaction system with 500 nm light, a characteristic emission band at around 605 nm of the photosensitizer is observed. The band intensity decreases dramatically with increasing TEOA content in the mixture of MeCN and H 2 O (Figure 4c), which indicates TEOA is an efficient quencher for excited states of [Ru(bpy) 3 ] 2+* , [37] and the recombination of excited charge carriers is substantially suppressed resulting in the decreased emission band intensity [38] …”

Polyoxometalate-based nanostructures for electrocatalytic and photocatalytic CO 2 reduction

“…108 In 2021, Draznieks reported three novel structures containing the {P4Mo6} anionic building unit connected by first-row transition-metal ions were successfully isolated by hydrothermal synthesis. 110 Ru-(bpy)-Co and Ru(bpy)-Mn exhibit a 1D structure with M(P4Mo6)2 (M = Co or Mn) sandwich-type POMs connected by additional Co(II) or Mn(II) ions. Conversely, Fe-Mn does not contain additional transition-metal complexes, and the POM units are interconnected via Fe-O(PO3) bonds, forming a compact 3D structure.…”

Photocatalytic CO2 Conversion Using Metal-Containing Coordination Polymers and Networks: Recent Developments in Material Design and Mechanistic Details

“…For the system containing Co(II)-POM units and [Ru(bpy)3] 2+ , DFT calculations could additionally show that these systems were unable to reduce CO2. The authors stated that Co(II) units dimerized while adsorbing an additional electron, making them inactive for CO2 reduction [61]. as a photosensitizer and TEOA as sacrificial agent using a POM system bearing Mn(II).…”

“…MLCT, ISC, MMCT, MC, and LSCT are abbreviations for metal-to-ligand charge transfer, intersystem crossing, metal-tometal charge transfer, metal-centered, and ligand-to-solvent charge transfer, respectively. Adapted with permission from Ref [61]…”