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.