Renewable light-driven photocatalytic CO₂ reduction (CO₂RR) has emerged as a promising strategy to mitigate greenhouse gas emissions while producing value-added chemicals and fuels. However, the efficiency of solar-driven photocatalytic systems remains limited by rapid recombination of photogenerated charge carriers, which represents a critical bottleneck for technological progress. Herein, ultrasmall WO₃ nanoclusters were successfully immobilized within the microporous framework of PCN-250 via a molecular cavity confinement strategy. Unlike pristine PCN-250, which primarily relies on monocomponent Fe³⁺ active sites, the immobilization of WO₃ nanoclusters enables the construction of a Z-scheme heterojunction. This unique architecture not only promotes efficient charge separation but also preserves the strong redox potentials of both components, thereby significantly enhancing the photocatalytic CO₂ reduction performance. HAADF-STEM analysis confirms the effective confinement and encapsulation of WO₃ nanoclusters within the PCN-250, where WO₃ species self-assemble into ultrathin nanoclusters with diameters ranging from 0.8 to 1.4 nm. UV-Vis spectroscopy reveals that this nanoscale encapsulation markedly broadens the optical absorption, extending the absorption edge to 800 nm and thus spanning the entire visible light region. Using H₂O vapor as a proton source, the optimized WO₃@PCN-250-2 (W@P-2) composite exhibits a CO₂ photoreduction rate of 516.07 μmol·g^-1, which is 9.1 times higher than that of pristine WO₃. Mechanistic studies indicate a Z-scheme charge transfer pathway at the WO₃/PCN-250 interface. In-situ FTIR spectroscopy identifies *COOH as the key intermediate during CO₂ reduction to CO. This work offers a valuable reference for designing Z-scheme heterojunctions toward efficient CO₂ photoreduction.

Key words: Ultrathin WO₃ nanocluster, PCN-250, Z-scheme heterojunction, CO₂ photoreduction, water vapor