Efficient and Durable Photochemical CO₂ Reduction by TiO₂-Immobilized Metal Porphyrin Catalysts

The advancement of molecular catalysts featuring 3d metals provides a sustainable alternative to noble-metal-based systems for CO₂-to-C1 fuel conversion; however, their widespread application remains limited by modest activity and selectivity. Herein, we report a dye-sensitized ternary hybrid photocatalyst composed of a visible-light-absorbing Ir(III) complex, a TiO₂ electron mediator, and a covalently anchored porphyrin catalyst incorporating 3d metals (Fe, Co, Ni, and Cu). Immobilization of the porphyrin via carboxylate groups enables strong electronic coupling with the TiO₂ scaffold, facilitating efficient multielectron accumulation and directional charge transfer. Under visible-light irradiation, the TiO₂-immobilized porphyrins (M-TCPP; M = Fe, Co, and Ni) exhibit markedly enhanced CO₂-to-CO conversion performance with turnover numbers exceeding 3000, surpassing their homogeneous analogs. Notably, Cu-TCPP shows distinct reactivity, favoring H₂ evolution over CO formation. Electrochemical analysis reveals a metal-dependent mechanistic divergence, in which Cu-TCPP undergoes protonation to form a Cu-H intermediate, whereas Fe-, Co-, and Ni-based porphyrins proceed through conventional M-COOH pathways. These findings highlight the synergistic role of TiO₂ in tuning interfacial electron transfer and promoting efficient CO₂ reduction using earth-abundant molecular catalysts.