Selective CO₂-to-CH₄ Photocatalytic Reduction via Spin-Modulation in a Metal─Carbon-Bonded MOF

The spin modulation of the electronic structure of photocatalysts offers a promising strategy to address selectivity challenges in photocatalysis. Herein, we present a novel ruthenium-carbon-bonded MOF (RuCMOF) and construct a heterojunction with Cu₂O nanoparticles for photocatalytic CO₂ reduction using water vapor. The optimized RuC-MOF/10%Cu₂O heterojunction achieves a remarkable CH₄ production rate of 663.6 µmol g^-1 h^-1with 96.7% selectivity under visible light irradiation. The apparent quantum yield reaches 4.6% at 400 nm. Experimental and theoretical studies reveal that the atomically dispersed ruthenium-alkynyl units of RuC-MOF and the heterostructure enable efficient photo-excited carrier separation/transfer. Moreover, the Cu₂O incorporation triggers a low-spin to high-spin transition in the Ru active centers of RuC-MOF, suppressing charge recombination through spin-selective electron transfer. Furthermore, the spin-state modulation also weakens *CO adsorption, lowers the energy barrier for *CHO formation and accelerates the rate-determining step. This work provides a new way for designing high-performance metal─carbon-bonded MOF photocatalysts through spin polarization engineering.