Unveiling the Mechanisms of Photocatalytic CO₂ Reduction Reaction in Semiconductor/Metal Complex Hybrid Photocatalyst

Hybrid catalysts combining semiconductors with molecular metal complexes have emerged as a promising strategy for efficient solar-to-chemical conversion under mild conditions; however, the mechanistic origin of their catalytic selectivity remains poorly understood. Here, we report a TiO₂/[Co(bpy)₃]²⁺ hybrid photocatalyst that exhibits significantly enhanced activity and selectivity for CO₂-to-CO conversion compared with pristine TiO₂. By combining electrochemical analysis, DFT, and TAS, we address consecutive elementary steps of the photocatalytic cycle: electrochemical analysis establishes the conditions for catalyst activation, DFT identifies the thermodynamic origin of CO₂ coordination selectivity at the activated center, and TAS provides supporting evidence for interfacial electron transfer dynamics under photocatalytic conditions. Specifically, DFT calculations show that CO₂ coordination is thermodynamically favored over proton adsorption, providing a basis for the suppressed hydrogen evolution. TAS observations are consistent with fast electron injection from TiO₂ to the Co-bpy complex and the formation of a long-lived transient species under CO₂ conditions, supporting the involvement of stabilized reduced intermediates in the proposed catalytic pathway. This interplay between interfacial charge transfer and CO₂-specific intermediate formation is consistent with the high selectivity toward CO production. These findings provide mechanistic insight into the roles of the semiconductor and molecular catalyst in hybrid photocatalysis.