Photocatalytic Conversion of CO₂ to Formate/CO by an (η<SUP>6</SUP>-<i>para</i>-Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO₂ Immobilization on the Catalytic Mechanism and Product Selectivity

초록

The catalytic efficacy of the monobipyridyl (eta(6)-para-Cymene)Ru(II) half-metallocene, [(p-Cym)Ru(bpy)Cl](+) was evaluated in both mixed homogeneous (dye + catalyst) and heterogeneous hybrid systems (dye/TiO2/Catalyst) for photochemical CO2 reduction. A series of homogeneous photolysis experiments revealed that the (p-Cym)Ru(II) catalyst engages in two competitive routes for CO2 reduction (CO2 to formate conversion via Ru-II-hydride vs CO2 to CO conversion through a Ru-II-COOH intermediate). The conversion activity and product selectivity were notably impacted by the pK(a) value and the concentration of the proton source added. When a more acidic TEOA additive was introduced, the half-metallocene Ru(II) catalyst leaned toward producing formate through the Ru-II-H mechanism, with a formate selectivity of 86%. On the other hand, in homogeneous catalysis with TFE additive, the CO2-to-formate conversion through Ru-II-H was less effective, yielding a more efficient CO2-to-CO conversion with a selectivity of >80% (TONformate of 140 and TONCO of 626 over 48 h). The preference between the two pathways was elucidated through an electrochemical mechanistic study, monitoring the fate of the metal-hydride intermediate. Compared to the homogeneous system, the TiO2-heterogenized (p-Cym)Ru(II) catalyst demonstrated enhanced and enduring performance, attaining TONs of 1000 for CO2-to-CO and 665 for CO2-to-formate.

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