Photocatalytic Conversion of CO₂ to Formate/CO by an (η⁶-para-Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO₂ Immobilization on the Catalytic Mechanism and Product Selectivity

The catalytic efficacy of the monobipyridyl (η⁶-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% (TON_formate of 140 and TON_CO 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.