Photocatalytic Conversion of CO 2 to Formate/CO by an (η 6 - para -Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO 2 Immobilization on the Catalytic Mechanism and Product Selectivity.

The catalytic efficacy of the monobipyridyl (η 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/TiO 2 /Catalyst) for photochemical CO 2 reduction. A series of homogeneous photolysis experiments revealed that the ( p -Cym)Ru(II) catalyst engages in two competitive routes for CO 2 reduction (CO 2 to formate conversion via Ru II -hydride vs CO 2 to CO conversion through a Ru II -COOH intermediate). The conversion activity and product selectivity were notably impacted by the p K 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 CO 2 -to-formate conversion through Ru II -H was less effective, yielding a more efficient CO 2 -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 TiO 2 -heterogenized ( p -Cym)Ru(II) catalyst demonstrated enhanced and enduring performance, attaining TONs of 1000 for CO 2 -to-CO and 665 for CO 2 -to-formate.